@article{zhou_kincaid_wang_annaberdiyev_ganesh_mitas_2024, title={A new generation of effective core potentials: Selected lanthanides and heavy elements}, volume={160}, ISSN={["1089-7690"]}, DOI={10.1063/5.0180057}, abstractNote={We construct correlation-consistent effective core potentials (ccECPs) for a selected set of heavy atoms and f elements that are currently of significant interest in materials and chemical applications, including Y, Zr, Nb, Rh, Ta, Re, Pt, Gd, and Tb. As is customary, ccECPs consist of spin–orbit (SO) averaged relativistic effective potential (AREP) and effective SO terms. For the AREP part, our constructions are carried out within a relativistic coupled-cluster framework while also taking into account objective function one-particle characteristics for improved convergence in optimizations. The transferability is adjusted using binding curves of hydride and oxide molecules. We address the difficulties encountered with f elements, such as the presence of large cores and multiple near-degeneracies of excited levels. For these elements, we construct ccECPs with core–valence partitioning that includes 4f subshell in the valence space. The developed ccECPs achieve an excellent balance between accuracy, size of the valence space, and transferability and are also suitable to be used in plane wave codes with reasonable energy cutoffs.}, number={8}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Zhou, Haihan and Kincaid, Benjamin and Wang, Guangming and Annaberdiyev, Abdulgani and Ganesh, Panchapakesan and Mitas, Lubos}, year={2024}, month={Feb} }
@article{huang_faizan_manzoor_brndiar_mitas_fabian_stich_2023, title={Colossal band gap response of single-layer phosphorene to strain predicted by quantum Monte Carlo}, volume={5}, ISSN={["2643-1564"]}, DOI={10.1103/PhysRevResearch.5.033223}, abstractNote={Straintronics is an emerging field enabling novel tuneable functionalities of electronic, optical, magnetic, or spin devices with advances being fuelled by new developments in van der Walls (vdW) heterostructure engineering and materials design. Here we show, using state-of-the-art quantum Monte Carlo (QMC) methods, that a single phosphorene monolayer exhibits outstanding straintronics functionalities due to discovered colossal strain tunability of its semiconducting electronic gap. First, we determine the equilibrium atomic structure that differs appreciably from available bulk phosphorene experimental data. That enables us to precisely analyze the quasiparticle band gaps for any uniaxial (armchair and zigzag) and biaxial strains which we describe by a quadrivariate paraboloid function of lattice and internal structure parameters. Using the fixed-node QMC calculations fitted by analytical formulas we localize the following excited state crossings: (i) between the direct ($\mathrm{\ensuremath{\Gamma}}\ensuremath{\rightarrow}\mathrm{\ensuremath{\Gamma}}$) and direct but reordered ($\mathrm{\ensuremath{\Gamma}}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Gamma}}}^{\ensuremath{'}}$) excitations that also imply substantial differences of corresponding transport properties; and (ii) between the direct $\mathrm{\ensuremath{\Gamma}}\ensuremath{\rightarrow}\mathrm{\ensuremath{\Gamma}}$ and indirect $\mathrm{\ensuremath{\Gamma}}\ensuremath{\rightarrow}\mathrm{X}$ excitations. Based on this highly accurate many-body treatment, we predict the gauge factor $\ensuremath{\approx}100$ meV/% and an unusual behavior with the band gap remaining direct even if strained by several percent. Consequently, we suggest there is a colossal band gap tunability window, larger by an order of magnitude when compared to quintessential straintronic materials such as ${\mathrm{MoS}}_{2}$. In addition, we ascertain that the ground state deformation energies exhibit an out-of plane negative Poisson's ratio and auxetic behavior.}, number={3}, journal={PHYSICAL REVIEW RESEARCH}, author={Huang, Y. and Faizan, Y. A. and Manzoor, M. and Brndiar, J. and Mitas, L. and Fabian, J. and Stich, I.}, year={2023}, month={Sep} }
@article{annaberdiyev_mandal_mitas_krogel_ganesh_2023, title={The role of electron correlations in the electronic structure of putative Chern magnet TbMn_{6}Sn_{6}}, volume={8}, ISSN={["2397-4648"]}, DOI={10.1038/s41535-023-00583-6}, abstractNote={AbstractA member of the RMn6Sn6 rare-earth family materials, TbMn6Sn6, recently showed experimental signatures of the realization of a quantum-limit Chern magnet. In this work, we use quantum Monte Carlo (QMC) and density functional theory with Hubbard U (DFT + U) calculations to examine the electronic structure of TbMn6Sn6. To do so, we optimize accurate, correlation-consistent pseudopotentials for Tb and Sn using coupled-cluster and configuration–interaction (CI) methods. We find that DFT + U and single-reference QMC calculations suffer from the same overestimation of the magnetic moments as meta-GGA and hybrid density functional approximations. Our findings point to the need for improved orbitals/wavefunctions for this class of materials, such as natural orbitals from CI, or for the inclusion of multi-reference effects that capture the static correlations for an accurate prediction of magnetic properties. DFT + U with Mn magnetic moments adjusted to the experiment predict the Dirac crossing in bulk to be close to the Fermi level, within ~120 meV, in agreement with the experiments. Our non-stoichiometric slab calculations show that the Dirac crossing approaches even closer to the Fermi level, suggesting the possible realization of Chern magnetism in this limit.}, number={1}, journal={NPJ QUANTUM MATERIALS}, author={Annaberdiyev, Abdulgani and Mandal, Subhasish and Mitas, Lubos and Krogel, Jaron T. and Ganesh, Panchapakesan}, year={2023}, month={Sep} }
@article{wang_kincaid_zhou_annaberdiyev_bennett_krogel_mitas_2022, title={A new generation of effective core potentials from correlated and spin-orbit calculations: Selected heavy elements}, volume={157}, ISSN={["1089-7690"]}, DOI={10.1063/5.0087300}, abstractNote={We introduce new correlation consistent effective core potentials (ccECPs) for the elements I, Te, Bi, Ag, Au, Pd, Ir, Mo, and W with 4d, 5d, 6s, and 6p valence spaces. These ccECPs are given as a sum of spin-orbit averaged relativistic effective potential (AREP) and effective spin–orbit (SO) terms. The construction involves several steps with increasing refinements from more simple to fully correlated methods. The optimizations are carried out with objective functions that include weighted many-body atomic spectra, norm-conservation criteria, and SO splittings. Transferability tests involve molecular binding curves of corresponding hydride and oxide dimers. The constructed ccECPs are systematically better and in a few cases on par with previous effective core potential (ECP) tables on all tested criteria and provide a significant increase in accuracy for valence-only calculations with these elements. Our study confirms the importance of the AREP part in determining the overall quality of the ECP even in the presence of sizable spin–orbit effects. The subsequent quantum Monte Carlo calculations point out the importance of accurate trial wave functions that, in some cases (mid-series transition elements), require treatment well beyond a single-reference.}, number={5}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Wang, Guangming and Kincaid, Benjamin and Zhou, Haihan and Annaberdiyev, Abdulgani and Bennett, M. Chandler and Krogel, Jaron T. and Mitas, Lubos}, year={2022}, month={Aug} }
@article{isaacs_shin_annaberdiyev_wolverton_mitas_benali_heinonen_2022, title={Assessing the accuracy of compound formation energies with quantum Monte Carlo}, volume={105}, ISSN={["2469-9969"]}, DOI={10.1103/PhysRevB.105.224110}, abstractNote={Accurately predicting the formation energy of a compound, which describes its thermodynamic stability, is a key challenge in materials physics. Here, we employ many-body quantum Monte Carlo (QMC) with single-reference trial functions to compute the formation energy of two electronically disparate compounds, the intermetallic VPt$_2$ and the semiconductor CuI, for which standard density functional theory (DFT) predictions using both the Perdew-Burke Ernzerhof (PBE) and the strongly constrained and appropriately normed (SCAN) density functional approximations deviate markedly from available experimental values. For VPt$_2$, we find an agreement between QMC, SCAN, and PBE0 estimates, which therefore remain in disagreement with the much less exothermic experimental value. For CuI, the QMC result agrees with neither SCAN nor PBE pointing towards DFT exchange-correlation biases, likely related to the localized Cu $3d$ electrons. Compared to the behavior of some density functional approximations within DFT, spin-averaged QMC exhibits a smaller but still appreciable deviation when compared to experiment. The QMC result is slightly improved by incorporating spin-orbit corrections for CuI and solid I$_2$, so that experiment and theory are brought into imperfect but reasonable agreement within about 120~meV/atom.}, number={22}, journal={PHYSICAL REVIEW B}, author={Isaacs, Eric B. and Shin, Hyeondeok and Annaberdiyev, Abdulgani and Wolverton, Chris and Mitas, Lubos and Benali, Anouar and Heinonen, Olle}, year={2022}, month={Jun} }
@article{kincaid_wang_zhou_mitas_2022, title={Correlation consistent effective core potentials for late 3d transition metals adapted for plane wave calculations}, volume={157}, ISSN={["1089-7690"]}, DOI={10.1063/5.0109098}, abstractNote={We construct a new modification of correlation consistent effective core potentials (ccECPs) for late 3d elements Cr–Zn with Ne-core that are adapted for efficiency and low energy cut-offs in plane wave calculations. The decrease in accuracy is rather minor, so that the constructions are in the same overall accuracy class as the original ccECPs. The resulting new constructions work with energy cut-offs at or below ≈400 Ry and, thus, make calculations of large systems with transition metals feasible for plane wave codes. We also provide the basic benchmarks for atomic spectra and molecular tests of this modified option that we denote as ccECP-soft.}, number={17}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Kincaid, Benjamin and Wang, Guangming and Zhou, Haihan and Mitas, Lubos}, year={2022}, month={Nov} }
@article{annaberdiyev_melton_wang_mitas_2022, title={Electronic structure of a-RuCl3 by fixed-node and fixed-phase diffusion Monte Carlo methods}, volume={106}, ISSN={["2469-9969"]}, DOI={10.1103/PhysRevB.106.075127}, abstractNote={Layered material $\alpha$-RuCl$_3$ has caught wide attention due to its possible realization of Kitaev's spin liquid and its electronic structure that involves the interplay of electron-electron correlations and spin-orbit effects. Several DFT$+U$ studies have suggested that both electron-electron correlations and spin-orbit effects are crucial for accurately describing the band gap. This work studies the importance of these two effects using fixed-node and fixed-phase diffusion Monte Carlo calculations both in spin-averaged and explicit spin-orbit formalisms. In the latter, the Slater-Jastrow trial function is constructed from two-component spin-orbitals using our recent quantum Monte Carlo (QMC) developments and thoroughly tested effective core potentials. Our results show that the gap in the ideal crystal is already accurately described by the spin-averaged case, with the dominant role being played by the magnetic ground state with significant exchange and electron correlation effects. We find qualitative agreement between hybrid DFT, DFT+$U$, and QMC. In addition, QMC results agree very well with available experiments, and we identify the values of exact Fock exchange mixing that provide comparable gaps. Explicit spin-orbit QMC calculations reveal that the effect of spin-orbit coupling on the gap is minor, of the order of 0.2 eV, which corresponds to the strength of the spin-orbit of the Ru atom.}, number={7}, journal={PHYSICAL REVIEW B}, author={Annaberdiyev, Abdulgani and Melton, Cody A. and Wang, Guangming and Mitas, Lubos}, year={2022}, month={Aug} }
@article{bennett_reboredo_mitas_krogel_2022, title={High Accuracy Transition Metal Effective Cores for the Many-Body Diffusion Monte Carlo Method}, ISSN={["1549-9626"]}, DOI={10.1021/acs.jctc.1c00992}, abstractNote={Practical applications of the real-space diffusion Monte Carlo (DMC) method require the removal of core electrons, where currently localization approximations of semilocal potentials are generally used in the projector. Accurate calculations of complex solids and large molecules demand minimizing the impact of approximated atomic cores. Prior works have shown that the errors from such approximations can be sizable in both finite and periodic systems. In this work, we show that a class of differential pseudopotentials, known as pseudo-Hamiltonians, can be constructed for the 3d transition metal atoms, entirely removing the need for any localization scheme in the DMC projector. As a proof of principle, we demonstrate the approach for the case of Co. In order to minimize errors in the pseudo-Hamiltonian at the many-body level, we generalize the recently proposed correlation-consistent pseudopotential generation scheme to successively close semilocal representations of the differential potentials. Our generation scheme successfully produces potentials tailored specifically for real space projector quantum Monte Carlo methods with low error at the many-body level, i.e., with many-body scattering properties very close to relativistic all-electron results. In particular, we show that the agreement with respect to atomic and molecular quantities reach chemical accuracy in many cases─on par with the most accurate semilocal pseudopotentials available. Further, our pseudo-Hamiltonian generation scheme utilizes standard quantum chemistry codes designed only to work with semilocal pseudopotentials, enabling straightforward generation of pseudo-Hamiltonians for additional elements in future works.}, journal={JOURNAL OF CHEMICAL THEORY AND COMPUTATION}, author={Bennett, M. Chandler and Reboredo, Fernando A. and Mitas, Lubos and Krogel, Jaron T.}, year={2022}, month={Jan} }
@article{mitas_annaberdiyev_2022, title={Weighted nodal domain averages of eigenstates for quantum Monte Carlo and beyond}, volume={557}, ISSN={["1873-4421"]}, DOI={10.1016/j.chemphys.2022.111483}, abstractNote={We study the nodal properties of many-body eigenstates of stationary Schrödinger equation that affect the accuracy of real-space quantum Monte Carlo calculations. In particular, we introduce weighted nodal domain averages that provide a new probe of nodal surfaces beyond the usual expectations. Particular choices for the weight function reveal, for example, that the difference between two arbitrary fermionic eigenvalues is given by the nodal hypersurface integrals normalized by overlaps with the bosonic ground state of the given Hamiltonian. Noninteracting and fully interacting Be atom with corresponding almost exact and approximate wave functions are used to illustrate several aspects of these concepts. Variational formulations that employ different weights are proposed for prospective improvement of nodes in variational and fixed-node diffusion Monte Carlo calculations.}, journal={CHEMICAL PHYSICS}, author={Mitas, Lubos and Annaberdiyev, Abdulgani}, year={2022}, month={May} }
@article{zhou_scemama_wang_annaberdiyev_kincaid_caffarel_mitas_2022, title={A quantum Monte Carlo study of systems with effective core potentials and node nonlinearities}, volume={554}, ISSN={["1873-4421"]}, DOI={10.1016/j.chemphys.2021.111402}, abstractNote={We study beryllium dihydride (BeH2) and acetylene (C2H2) molecules using real-space diffusion Monte Carlo (DMC) method. The molecules serve as perhaps the simplest prototypes that illustrate the difficulties with biases in the fixed-node DMC calculations that might appear with the use of effective core potentials (ECPs) or other nonlocal operators. This is especially relevant for the recently introduced correlation consistent ECPs (ccECPs) for 2s2p elements. Corresponding ccECPs exhibit deeper potential functions due to higher fidelity to all-electron counterparts, which could lead to larger local energy fluctuations. We point out that the difficulties stem from issues that are straightforward to address by upgrades of basis sets, use of T-moves for nonlocal terms, inclusion of a few configurations into the trial function and similar. The resulting accuracy corresponds to the ccECP target (chemical accuracy) and it is in consistent agreement with independent correlated calculations. Further possibilities for upgrading the reliability of the DMC algorithm and considerations for better adapted and more robust Jastrow factors are discussed as well.}, journal={CHEMICAL PHYSICS}, author={Zhou, Haihan and Scemama, Anthony and Wang, Guangming and Annaberdiyev, Abdulgani and Kincaid, Benjamin and Caffarel, Michel and Mitas, Lubos}, year={2022}, month={Feb} }
@article{annaberdiyev_wang_melton_bennett_mitas_2021, title={Cohesion and excitations of diamond-structure silicon by quantum Monte Carlo: Benchmarks and control of systematic biases}, volume={103}, ISSN={["2469-9969"]}, DOI={10.1103/PhysRevB.103.205206}, abstractNote={We have carried out quantum Monte Carlo (QMC) calculations of silicon crystal focusing on the accuracy and systematic biases that affect the electronic structure characteristics. The results show that 64 and 216 atom supercells provide an excellent consistency for extrapolated energies per atom in the thermodynamic limit for ground, excited, and ionized states. We have calculated the ground state cohesion energy with both systematic and statistical errors below ≈0.05 eV. The ground state exhibits a fixed-node error of only 1.3(2)% of the correlation energy, suggesting an unusually high accuracy of the corresponding single-reference trial wave function. We obtain a very good agreement between optical and quasiparticle gaps that affirms the marginal impact of excitonic effects. Our most accurate results for band gaps differ from the experiments by about 0.2 eV. This difference is assigned to a combination of residual finite-size and fixed-node errors. We have estimated the crystal Fermi level referenced to vacuum that enabled us to calculate the edges of valence and conduction bands in agreement with experiments.}, number={20}, journal={PHYSICAL REVIEW B}, author={Annaberdiyev, Abdulgani and Wang, Guangming and Melton, Cody A. and Bennett, M. Chandler and Mitas, Lubos}, year={2021}, month={May} }
@article{annaberdiyev_melton_bennett_wang_mitas_2020, title={Accurate Atomic Correlation and Total Energies for Correlation Consistent Effective Core Potentials}, volume={16}, ISSN={["1549-9626"]}, DOI={10.1021/acs.jctc.9b00962}, abstractNote={Very recently, we introduced a set of correlation consistent effective core potentials (ccECPs) constructed within full many-body approaches. By employing significantly more accurate correlated approaches we were able to reach a new level of accuracy for the resulting effective core Hamiltonians. We also strived for simplicity of use and easy transferability into a variety of electronic structure methods in quantum chemistry and condensed matter physics. Here, as a reference for future use, we present exact or nearly-exact total energy calculations for these ccECPs. The calculations cover H-Kr elements and are based on the state-of-the-art configuration interaction (CI), coupled-cluster (CC), and quantum Monte Carlo (QMC) calculations with systematically eliminated/improved errors. In particular, we carry out full CI/CCSD(T)/CCSDT(Q) calculations with cc-pVnZ with up to n=6 basis sets and we estimate the complete basis set limits. Using combinations of these approaches, we achieved an accuracy of ≈ 1-10 mHa for K-Zn atoms and ≈ 0.1-0.3 mHa for all other elements - within about 1% or better of the ccECP total correlation energies. We also estimate the corresponding kinetic energies within the feasible limit of full CI calculations. In order to provide data for QMC calculations, we include fixed-node diffusion Monte Carlo energies for each element that give quantitative insights into the fixed-node biases for single-reference trial wave functions. The results offer a clear benchmark for future high accuracy calculations in a broad variety of correlated wave function methods such as CI and CC as well is in stochastic approaches such as real space sampling QMC.}, number={3}, journal={JOURNAL OF CHEMICAL THEORY AND COMPUTATION}, author={Annaberdiyev, Abdulgani and Melton, Cody A. and Bennett, M. Chandler and Wang, Guangming and Mitas, Lubos}, year={2020}, month={Mar}, pages={1482–1502} }
@article{wang_annaberdiyev_mitas_2020, title={Binding and excitations in SixHy molecular systems using quantum Monte Carlo}, volume={153}, ISSN={["1089-7690"]}, DOI={10.1063/5.0022814}, abstractNote={We present high-accuracy correlated calculations of small SixHy molecular systems in both the ground and excited states. We employ quantum Monte Carlo (QMC) together with a variety of many-body wave function approaches based on basis set expansions. The calculations are carried out in a valence-only framework using recently derived correlation consistent effective core potentials. Our primary goal is to understand the fixed-node diffusion QMC errors in both the ground and excited states with single-reference trial wave functions. Using a combination of methods, we demonstrate the very high accuracy of the QMC atomization energies being within ≈0.07 eV or better when compared with essentially exact results. By employing proper choices for trial wave functions, we have found that the fixed-node QMC biases for total energies are remarkably uniform ranging between 1% and 3.5% with absolute values at most ≈0.2 eV across the systems and several types of excitations such as singlets and triplets as well as low-lying and Rydberg-like states. Our results further corroborate that Si systems, and presumably also related main group IV and V elements of the periodic table (Ge, Sn, etc), exhibit some of the lowest fixed-node biases found in valence-only electronic structure QMC calculations.}, number={14}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Wang, Guangming and Annaberdiyev, Abdulgani and Mitas, Lubos}, year={2020}, month={Oct} }
@article{dubecky_karlicky_minarik_mitas_2020, title={Fundamental gap of fluorographene by many-body GW and fixed-node diffusion Monte Carlo methods}, volume={153}, ISSN={["1089-7690"]}, DOI={10.1063/5.0030952}, abstractNote={Fluorographene (FG) is a promising graphene-derived material with a large bandgap. Currently existing predictions of its fundamental gap (Δf) and optical gap (Δopt) significantly vary when compared with experiment. We provide here an ultimate benchmark of Δf for FG by many-body GW and fixed-node diffusion Monte Carlo (FNDMC) methods. Both approaches independently arrive at Δf ≈ 7.1 ± 0.1 eV. In addition, the Bethe–Salpeter equation enabled us to determine the first exciton binding energy, Eb = 1.92 eV. We also point to the possible misinterpretation problem of the results obtained for gaps of solids by FNDMC with single-reference trial wave functions of Bloch orbitals. We argue why instead of Δopt, in the thermodynamic limit, such an approach results in energy differences that rather correspond to Δf, and we also outline conditions when this case actually applies.}, number={18}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Dubecky, Matus and Karlicky, Frantisek and Minarik, Stanislav and Mitas, Lubos}, year={2020}, month={Nov} }
@article{melton_mitas_2020, title={Many-body electronic structure of LaScO3 by real-space quantum Monte Carlo}, volume={102}, ISSN={["2469-9969"]}, DOI={10.1103/PhysRevB.102.045103}, abstractNote={We present real space quantum Monte Carlo (QMC) calculations of the scandate LaScO$_3$ that proved to be challenging for traditional electronic structure approaches due to strong correlation effects resulting in inaccurate band gaps from DFT and $GW$ methods when compared with existing experimental data. Besides calculating an accurate QMC band gap corrected for supercell size biases and in agreement with numerous experiments, we also predict the cohesive energy of the crystal using the standard fixed-node QMC without any empirical or non-variational parameters. We show that promotion (optical) gap and fundamental gap agree with each other illustrating a clear absence of significant excitonic effects in the ideal crystal. We obtained these results in perfect consistency in two independent tracks that employ different basis sets (plane wave vs. localized gaussians), different codes for generating orbitals (\textsc{Quantum Espresso} vs. \textsc{Crystal}), different QMC codes (\textsc{Qmcpack} vs. \textsc{Qwalk}) and different high-accuracy pseudopotentials (ccECPs vs. Troullier-Martins) presenting the maturity and consistency of QMC methodology and tools for studies of strongly correlated problems.}, number={4}, journal={PHYSICAL REVIEW B}, author={Melton, Cody A. and Mitas, Lubos}, year={2020}, month={Jul} }
@article{kent_annaberdiyev_benali_bennett_borda_doak_hao_jordan_krogel_kylanpaa_et al._2020, title={QMCPACK: Advances in the development, efficiency, and application of auxiliary field and real-space variational and diffusion quantum Monte Carlo}, volume={152}, ISSN={["1089-7690"]}, DOI={10.1063/5.0004860}, abstractNote={We review recent advances in the capabilities of the open source ab initio Quantum Monte Carlo (QMC) package QMCPACK and the workflow tool Nexus used for greater efficiency and reproducibility. The auxiliary field QMC (AFQMC) implementation has been greatly expanded to include k-point symmetries, tensor-hypercontraction, and accelerated graphical processing unit (GPU) support. These scaling and memory reductions greatly increase the number of orbitals that can practically be included in AFQMC calculations, increasing the accuracy. Advances in real space methods include techniques for accurate computation of bandgaps and for systematically improving the nodal surface of ground state wavefunctions. Results of these calculations can be used to validate application of more approximate electronic structure methods, including GW and density functional based techniques. To provide an improved foundation for these calculations, we utilize a new set of correlation-consistent effective core potentials (pseudopotentials) that are more accurate than previous sets; these can also be applied in quantum-chemical and other many-body applications, not only QMC. These advances increase the efficiency, accuracy, and range of properties that can be studied in both molecules and materials with QMC and QMCPACK.}, number={17}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Kent, P. R. C. and Annaberdiyev, Abdulgani and Benali, Anouar and Bennett, M. Chandler and Borda, Edgar Josue Landinez and Doak, Peter and Hao, Hongxia and Jordan, Kenneth D. and Krogel, Jaron T. and Kylanpaa, Ilkka and et al.}, year={2020}, month={May} }
@article{wang_annaberdiyev_melton_bennett_shulenburger_mitas_2019, title={A new generation of effective core potentials from correlated calculations: 4s and 4p main group elements and first row additions}, volume={151}, ISSN={["1089-7690"]}, DOI={10.1063/1.5121006}, abstractNote={Recently, we developed a new method for generating effective core potentials (ECPs) using valence energy isospectrality with explicitly correlated all-electron (AE) excitations and norm-conservation criteria. We apply this methodology to the 3rd-row main group elements, creating new correlation consistent ECPs (ccECPs) and also deriving additional ECPs to complete the ccECP table for H–Kr. For K and Ca, we develop Ne-core ECPs, and for the 4p main group elements, we construct [Ar]3d10-core potentials. Scalar relativistic effects are included in their construction. Our ccECPs reproduce AE spectra with significantly better accuracy than many existing pseudopotentials and show better overall consistency across multiple properties. The transferability of ccECPs is tested on monohydride and monoxide molecules over a range of molecular geometries. For the constructed ccECPs, we also provide optimized DZ-6Z valence Gaussian basis sets.}, number={14}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Wang, Guangming and Annaberdiyev, Abdulgani and Melton, Cody A. and Bennett, M. Chandler and Shulenburger, Luke and Mitas, Lubos}, year={2019}, month={Oct} }
@article{kulahlioglu_mitas_2019, title={A quantum Monte Carlo study of the molybdenum dimer (Mo-2)}, volume={1170}, ISSN={["1872-7999"]}, DOI={10.1016/j.comptc.2019.112642}, abstractNote={We have studied the molybdenum dimer (Mo2) system. The binding energy was calculated by means of the fixed-node DMC (FN-DMC) method. The Slater part of the trial wave function was constructed by the Selected-CI method by using the orbitals generated by the KS-DFT method with a hybrid meta-GGA exchange and correlation functional, TPSSh. We also carried out CCSD(T) calculations which were subsequently extrapolated to the complete basis set (CBS) limit. The results are presented.}, journal={COMPUTATIONAL AND THEORETICAL CHEMISTRY}, author={Kulahlioglu, Adem Halil and Mitas, Lubos}, year={2019}, month={Dec} }
@article{frank_derian_tokar_mitas_fabian_stich_2019, title={Many-Body Quantum Monte Carlo Study of 2D Materials: Cohesion and Band Gap in Single-Layer Phosphorene}, volume={9}, ISSN={["2160-3308"]}, DOI={10.1103/PhysRevX.9.011018}, abstractNote={The quantum Monte Carlo (QMC) method is applied to obtain the fundamental (quasiparticle) electronic band gap Δf of a semiconducting two-dimensional phosphorene whose optical and electronic properties fill the void between graphene and 2D transition-metal dichalcogenides. Similarly to other 2D materials, the electronic structure of phosphorene is strongly influenced by reduced screening, making it challenging to obtain reliable predictions by single-particle density-functional methods. Advanced GW techniques, which include many-body effects as perturbative corrections, are hardly consistent with each other, predicting the band gap of phosphorene with a spread of almost 1 eV, from 1.6 to 2.4 eV. Our QMC results, from infinite periodic superlattices as well as from finite clusters, predict Δf to be about 2.4 eV, indicating that available GW results are systematically underestimating the gap. Using the recently uncovered universal scaling between the exciton binding energy and Δf, we predict the optical gap of about 1.7 eV that can be directly related to measurements even on encapsulated samples due to its robustness against dielectric environment. The QMC gaps are indeed consistent with recent experiments based on optical absorption and photoluminescence excitation spectroscopy. We also predict the cohesion of phosphorene to be only slightly smaller than that of the bulk crystal. Our investigations not only benchmark GW methods and experiments, but also open the field of 2D electronic structure to computationally intensive but highly predictive QMC methods which include many-body effects such as electronic correlations and van der Waals interactions explicitly.}, number={1}, journal={PHYSICAL REVIEW X}, author={Frank, T. and Derian, R. and Tokar, K. and Mitas, L. and Fabian, J. and Stich, I.}, year={2019}, month={Jan} }
@article{melton_bennett_mitas_2019, title={Projector quantum Monte Carlo with averaged vs explicit spin-orbit effects: Applications to tungsten molecular systems}, volume={128}, ISSN={["1879-2553"]}, DOI={10.1016/j.jpcs.2017.12.033}, abstractNote={We present a recently developed projector quantum Monte Carlo method for calculations of electronic structure in systems with spin-orbit interactions. The method solves for many-body eigenstates in the presence of spin-orbit using the fixed-phase approximation. The trial wave function is built from two-component spinors and explicit Jastrow correlation factors while the core electrons are eliminated by relativistic effective core potentials with explicit spin-orbit terms. We apply this method to WO and W2 molecules that enables us to build multi-reference wave functions and analyze the impact of both electron correlations and the spin-orbit terms.}, journal={JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS}, author={Melton, Cody A. and Bennett, M. Chandler and Mitas, Lubos}, year={2019}, month={May}, pages={367–373} }
@article{dubecky_jurecka_mitas_ditte_fanta_2019, title={Toward Accurate Hydrogen Bonds by Scalable Quantum Monte Carlo}, volume={15}, ISSN={["1549-9626"]}, DOI={10.1021/acs.jctc.9b00096}, abstractNote={Single-determinant (SD) fixed-node diffusion Monte Carlo (FNDMC) gains popularity as a benchmark method scalable to large noncovalent systems, although its accuracy limits are not yet fully mapped out. We report on an interesting example of significant SD FNDMC accuracy variations in middle-sized hydrogen-bonded dimer complexes, formic acid (FA) vs methanediol (MD), distinct by the maximum bond order (2 vs 1). While the traditional SD FNDMC schemes based on bias cancellation are capable of achieving benchmark (2%) accuracy for MD, this has not been the case for FA. We identify the leading systematic error source in energy differences and show that suitably designed Jastrow factors enable SD FNDMC to reach the reference accuracy for FA. This work clearly illustrates the varying accuracy of the present-day SD FNDMC at the 0.1 kcal/mol scale for a particular set of systems but also points out promising routes toward alleviation of these shortcomings, still within the single-reference framework.}, number={6}, journal={JOURNAL OF CHEMICAL THEORY AND COMPUTATION}, author={Dubecky, Matus and Jurecka, Petr and Mitas, Lubos and Ditte, Matej and Fanta, Roman}, year={2019}, month={Jun}, pages={3552–3557} }
@article{bennett_wang_annaberdiyev_melton_shulenburger_mitas_2018, title={A new generation of effective core potentials from correlated calculations: 2nd row elements}, volume={149}, ISSN={["1089-7690"]}, DOI={10.1063/1.5038135}, abstractNote={Very recently, we have introduced correlation consistent effective core potentials (ccECPs) derived from many-body approaches with the main target being their use in explicitly correlated methods, while still usable in mainstream approaches. The ccECPs are based on reproducing excitation energies for a subset of valence states, namely, achieving near-isospectrality between the original and pseudo Hamiltonians. In addition, binding curves of dimer molecules were used for refinement and overall improvement of transferability over a range of bond lengths. Here we apply similar ideas to the 2nd row elements and study several aspects of the constructions in order to find the high accuracy solutions within the chosen ccECP forms with 3s, 3p valence space (Ne-core). Our new constructions exhibit accurate low-lying atomic excitations and equilibrium molecular bonds (on average within ≈0.03 eV and 3 mÅ); however, the errors for Al and Si oxide molecules at short bond lengths are notably larger for both ours and existing effective core potentials. Assuming this limitation, our ccECPs show a systematic balance between the criteria of atomic spectra accuracy and transferability for molecular bonds. In order to provide another option with much higher uniform accuracy, we also construct He-core ccECPs for the whole 2nd row with typical discrepancies of ≈0.01 eV or smaller.}, number={10}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Bennett, M. Chandler and Wang, Guangming and Annaberdiyev, Abdulgani and Melton, Cody A. and Shulenburger, Luke and Mitas, Lubos}, year={2018}, month={Sep} }
@article{annaberdiyev_wang_melton_bennett_shulenburger_mitas_2018, title={A new generation of effective core potentials from correlated calculations: 3d transition metal series}, volume={149}, ISSN={["1089-7690"]}, DOI={10.1063/1.5040472}, abstractNote={Recently, we have introduced a new generation of effective core potentials (ECPs) designed for accurate correlated calculations but equally useful for a broad variety of approaches. The guiding principle has been the isospectrality of all-electron and ECP Hamiltonians for a subset of valence many-body states using correlated, nearly-exact calculations. Here we present such ECPs for the 3d transition series Sc to Zn with Ne-core, i.e., with semi-core 3s and 3p electrons in the valence space. Besides genuine many-body accuracy, the operators are simple, being represented by a few gaussians per symmetry channel with resulting potentials that are bounded everywhere. The transferability is checked on selected molecular systems over a range of geometries. The ECPs show a high overall accuracy with valence spectral discrepancies typically ≈0.01-0.02 eV or better. They also reproduce binding curves of hydride and oxide molecules typically within 0.02-0.03 eV deviations over the full non-dissociation range of interatomic distances.}, number={13}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Annaberdiyev, Abdulgani and Wang, Guangming and Melton, Cody A. and Bennett, M. Chandler and Shulenburger, Luke and Mitas, Lubos}, year={2018}, month={Oct} }
@misc{alberi_nardelli_zakutayev_mitas_curtarolo_jain_fornari_marzari_takeuchi_green_et al._2019, title={The 2019 materials by design roadmap}, volume={52}, ISSN={["1361-6463"]}, DOI={10.1088/1361-6463/aad926}, abstractNote={Advances in renewable and sustainable energy technologies critically depend on our ability to design and realize materials with optimal properties. Materials discovery and design efforts ideally involve close coupling between materials prediction, synthesis and characterization. The increased use of computational tools, the generation of materials databases, and advances in experimental methods have substantially accelerated these activities. It is therefore an opportune time to consider future prospects for materials by design approaches. The purpose of this Roadmap is to present an overview of the current state of computational materials prediction, synthesis and characterization approaches, materials design needs for various technologies, and future challenges and opportunities that must be addressed. The various perspectives cover topics on computational techniques, validation, materials databases, materials informatics, high-throughput combinatorial methods, advanced characterization approaches, and materials design issues in thermoelectrics, photovoltaics, solid state lighting, catalysts, batteries, metal alloys, complex oxides and transparent conducting materials. It is our hope that this Roadmap will guide researchers and funding agencies in identifying new prospects for materials design.}, number={1}, journal={JOURNAL OF PHYSICS D-APPLIED PHYSICS}, author={Alberi, Kirstin and Nardelli, Marco Buongiorno and Zakutayev, Andriy and Mitas, Lubos and Curtarolo, Stefano and Jain, Anubhav and Fornari, Marco and Marzari, Nicola and Takeuchi, Ichiro and Green, Martin L. and et al.}, year={2019}, month={Jan} }
@article{bennett_melton_annaberdiyev_wang_shulenburger_mitas_2017, title={A new generation of effective core potentials for correlated calculations}, volume={147}, ISSN={["1089-7690"]}, DOI={10.1063/1.4995643}, abstractNote={We outline ideas on desired properties for a new generation of effective core potentials (ECPs) that will allow valence-only calculations to reach the full potential offered by recent advances in many-body wave function methods. The key improvements include consistent use of correlated methods throughout ECP constructions and improved transferability as required for an accurate description of molecular systems over a range of geometries. The guiding principle is the isospectrality of all-electron and ECP Hamiltonians for a subset of valence states. We illustrate these concepts on a few first- and second-row atoms (B, C, N, O, S), and we obtain higher accuracy in transferability than previous constructions while using semi-local ECPs with a small number of parameters. In addition, the constructed ECPs enable many-body calculations of valence properties with higher (or same) accuracy than their all-electron counterparts with uncorrelated cores. This implies that the ECPs include also some of the impacts of core-core and core-valence correlations on valence properties. The results open further prospects for ECP improvements and refinements.}, number={22}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Bennett, M. Chandler and Melton, Cody A. and Annaberdiyev, Abdulgani and Wang, Guangming and Shulenburger, Luke and Mitas, Lubos}, year={2017}, month={Dec} }
@article{bennett_kulahlioglu_mitas_2017, title={A quantum Monte Carlo study of mono(benzene) TM and bis(benzene) TM systems}, volume={667}, ISSN={["1873-4448"]}, DOI={10.1016/j.cplett.2016.11.032}, abstractNote={We present a study of mono(benzene)TM and bis(benzene)TM systems, where TM={Mo,W}. We calculate the binding energies by quantum Monte Carlo (QMC) approaches and compare the results with other methods and available experiments. The orbitals for the determinantal part of each trial wave function were generated from several types of DFT in order to optimize for fixed-node errors. We estimate and compare the size of the fixed-node errors for both the Mo and W systems with regard to the electron density and degree of localization in these systems. For the W systems we provide benchmarking results of the binding energies, given that experimental data is not available.}, journal={CHEMICAL PHYSICS LETTERS}, author={Bennett, M. Chandler and Kulahlioglu, A. H. and Mitas, L.}, year={2017}, month={Jan}, pages={74–78} }
@article{melton_mitas_2017, title={Quantum Monte Carlo with variable spins: Fixed-phase and fixed-node approximations}, volume={96}, ISSN={["2470-0053"]}, DOI={10.1103/physreve.96.043305}, abstractNote={We study several aspects of the recently introduced fixed-phase spinor diffusion Monte Carlo method, in particular, its relation to the fixed-node method and its potential use as a general approach for electronic structure calculations. We illustrate constructions of spinor-based wave functions with the full space-spin symmetry without assigning up or down spin labels to particular electrons, effectively "complexifying" even ordinary real-valued wave functions for Hamiltonians without spin terms. Interestingly, with proper choice of the simulation parameters and spin variables, such fixed-phase calculations enable one to reach also the fixed-node limit. The fixed-phase approximation has several desirable properties when compared to the fixed-node approximation. The fixed-phase solution provides a straightforward interpretation as the lowest bosonic state in a given effective potential generated by the many-body approximate phase, whereas nodal boundary conditions are defined through less intuitive and complicated hypersurfaces with one dimension less than the original configuration space. In addition, the divergences of the local energy and drift at real wave function nodes are smoothed out to lower dimensionality when the wave function is complexified, thus decreasing the variation of sampled quantities and eliminating artificial nodal domain issues that can occur in the fixed-node formalism. We illustrate some of these properties on calculations of selected first-row systems that recover the fixed-node results with quantitatively similar levels of the corresponding biases. At the same time, the fixed-phase approach opens new possibilities for more general trial wave functions with further opportunities for increasing accuracy in practical calculations.}, number={4}, journal={PHYSICAL REVIEW E}, author={Melton, Cody A. and Mitas, Lubos}, year={2017}, month={Oct} }
@article{tokar_derian_mitas_stich_2016, title={Charged vanadium-benzene multidecker clusters: DFT and quantum Monte Carlo study}, volume={144}, ISSN={["1089-7690"]}, DOI={10.1063/1.4941085}, abstractNote={Using explicitly correlated fixed-node quantum Monte Carlo and density functional theory (DFT) methods, we study electronic properties, ground-state multiplets, ionization potentials, electron affinities, and low-energy fragmentation channels of charged half-sandwich and multidecker vanadium-benzene systems with up to 3 vanadium atoms, including both anions and cations. It is shown that, particularly in anions, electronic correlations play a crucial role; these effects are not systematically captured with any commonly used DFT functionals such as gradient corrected, hybrids, and range-separated hybrids. On the other hand, tightly bound cations can be described qualitatively by DFT. A comparison of DFT and quantum Monte Carlo provides an in-depth understanding of the electronic structure and properties of these correlated systems. The calculations also serve as a benchmark study of 3d molecular anions that require a balanced many-body description of correlations at both short- and long-range distances.}, number={6}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Tokar, K. and Derian, R. and Mitas, L. and Stich, I.}, year={2016}, month={Feb} }
@inproceedings{melton_mitas_2016, title={Fixed-node and fixed-phase approximations and their relationship to variable spins in quantum monte carlo}, volume={1234}, DOI={10.1021/bk-2016-1234.ch001}, abstractNote={We compare the fixed-phase approximation with the better known, but closely related fixed-node approximation on several testing examples. We found that both approximations behave very similarly with the fixed-phase results being very close to the fixed-node method whenever nodes/phase were of high and comparable accuracy. The fixed-phase exhibited larger biases when the trial wave functions errors in the nodes/phase were intentionally driven to unrealistically large values. We also present a formalism that enables to describe wave functions with the full antisymmetry in spin-spatial degrees of freedom using our recently developed method for systems with spins as fully quantum variables. This opens new possibilities for simulations of fermionic systems in the fixed-phase approximation formalism.}, booktitle={Recent progress in quantum monte carlo}, author={Melton, C. A. and Mitas, L.}, year={2016}, pages={1–13} }
@article{niu_dinan_tirukkovalur_benali_kim_mitas_wagner_sadayappan_2016, title={Global-view coefficients: a data management solution for parallel quantum Monte Carlo applications}, volume={28}, ISSN={["1532-0634"]}, DOI={10.1002/cpe.3748}, abstractNote={SummaryQuantum Monte Carlo (QMC) applications perform simulation with respect to an initial state of the quantum mechanical system, which is often captured by using a cubic B‐spline basis. This representation is stored as a read‐only table of coefficients and accesses to the table are generated at random as part of the Monte Carlo simulation. Current QMC applications, such as QWalk and QMCPACK, replicate this table at every process or node, which limits scalability because increasing the number of processors does not enable larger systems to be run. We present a partitioned global address space approach to transparently managing this data using Global Arrays in a manner that allows the memory of multiple nodes to be aggregated. We develop an automated data management system that significantly reduces communication overheads, enabling new capabilities for QMC codes. Experimental results with QWalk and QMCPACK demonstrate the effectiveness of the data management system. Copyright © 2016 John Wiley & Sons, Ltd.}, number={13}, journal={CONCURRENCY AND COMPUTATION-PRACTICE & EXPERIENCE}, author={Niu, Qingpeng and Dinan, James and Tirukkovalur, Sravya and Benali, Anouar and Kim, Jeongnim and Mitas, Lubos and Wagner, Lucas and Sadayappan, P.}, year={2016}, month={Sep}, pages={3655–3671} }
@misc{dubecky_mitas_jurecka_2016, title={Noncovalent Interactions by Quantum Monte Carlo}, volume={116}, ISSN={["1520-6890"]}, DOI={10.1021/acs.chemrev.5b00577}, abstractNote={Quantum Monte Carlo (QMC) is a family of stochastic methods for solving quantum many-body problems such as the stationary Schrödinger equation. The review introduces basic notions of electronic structure QMC based on random walks in real space as well as its advances and adaptations to systems with noncovalent interactions. Specific issues such as fixed-node error cancellation, construction of trial wave functions, and efficiency considerations that allow for benchmark quality QMC energy differences are described in detail. Comprehensive overview of articles covers QMC applications to systems with noncovalent interactions over the last three decades. The current status of QMC with regard to efficiency, applicability, and usability by nonexperts together with further considerations about QMC developments, limitations, and unsolved challenges are discussed as well.}, number={9}, journal={CHEMICAL REVIEWS}, author={Dubecky, Matus and Mitas, Lubos and Jurecka, Petr}, year={2016}, month={May}, pages={5188–5215} }
@article{melton_bennett_mitas_2016, title={Quantum Monte Carlo with variable spins}, volume={144}, ISSN={["1089-7690"]}, DOI={10.1063/1.4954726}, abstractNote={We investigate the inclusion of variable spins in electronic structure quantum Monte Carlo, with a focus on diffusion Monte Carlo with Hamiltonians that include spin-orbit interactions. Following our previous introduction of fixed-phase spin-orbit diffusion Monte Carlo, we thoroughly discuss the details of the method and elaborate upon its technicalities. We present a proof for an upper-bound property for complex nonlocal operators, which allows for the implementation of T-moves to ensure the variational property. We discuss the time step biases associated with our particular choice of spin representation. Applications of the method are also presented for atomic and molecular systems. We calculate the binding energies and geometry of the PbH and Sn2 molecules, as well as the electron affinities of the 6p row elements in close agreement with experiments.}, number={24}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Melton, Cody A. and Bennett, M. Chandler and Mitas, Lubos}, year={2016}, month={Jun} }
@article{melton_zhu_guo_ambrosetti_pederiva_mitas_2016, title={Spin-orbit interactions in electronic structure quantum Monte Carlo methods}, volume={93}, ISSN={["2469-9934"]}, DOI={10.1103/physreva.93.042502}, abstractNote={We develop generalization of the fixed-phase diffusion Monte Carlo method for Hamiltonians which explicitly depend on particle spins such as for spin-orbit interactions. The method is formulated in zero variance manner and is similar to treatment of nonlocal operators in commonly used static- spin calculations. Tests on atomic and molecular systems show that it is very accurate, on par with the fixed-node method. This opens electronic structure quantum Monte Carlo methods to a vast research area of quantum phenomena in which spin-related interactions play an important role.}, number={4}, journal={PHYSICAL REVIEW A}, author={Melton, Cody A. and Zhu, Minyi and Guo, Shi and Ambrosetti, Alberto and Pederiva, Francesco and Mitas, Lubos}, year={2016}, month={Apr} }
@article{rasch_mitas_2015, title={Fixed-node diffusion Monte Carlo method for lithium systems}, volume={92}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.92.045122}, abstractNote={We study lithium systems over a range of number of atoms, e.g., atomic anion, dimer, metallic cluster, and body-centered cubic crystal by the diffusion Monte Carlo method. The calculations include both core and valence electrons in order to avoid any possible impact by pseudo potentials. The focus of the study is the fixed-node errors, and for that purpose we test several orbital sets in order to provide the most accurate nodal hyper surfaces. We compare our results to other high accuracy calculations wherever available and to experimental results so as to quantify the the fixed-node errors. The results for these Li systems show that fixed-node quantum Monte Carlo achieves remarkably high accuracy total energies and recovers 97-99 % of the correlation energy.}, number={4}, journal={PHYSICAL REVIEW B}, author={Rasch, K. M. and Mitas, L.}, year={2015}, month={Jul} }
@article{ambrosetti_silvestrelli_pederiva_mitas_toigo_2015, title={Repulsive atomic Fermi gas with Rashba spin-orbit coupling: A quantum Monte Carlo study}, volume={91}, DOI={10.1103/physreva.91.053622}, abstractNote={Alberto Ambrosetti,1,* Pier Luigi Silvestrelli,1 Francesco Pederiva,2,3 Lubos Mitas,4 and Flavio Toigo1 1Dipartimento di Fisica, University of Padova, via Marzolo 8, I-35131, Padova, Italy 2Dipartimento di Fisica e LISC, Universita di Trento, Via Sommarive 14, I-38123, Povo, Trento, Italy 3Trento Institute for Fundamental Physics and Applications, Trento, Italy 4Department of Physics, North Carolina State University, Raleigh, North Carolina 27695-8202, USA (Received 13 November 2014; published 26 May 2015)}, number={5}, journal={Physical Review. A}, author={Ambrosetti, A. and Silvestrelli, P. L. and Pederiva, F. and Mitas, L. and Toigo, F.}, year={2015} }
@article{kulahlioglu_mitas_2014, title={A quantum Monte Carlo study of zinc-porphyrin: Vertical excitation between the singlet ground state and the lowest-lying singlet excited state}, volume={1046}, ISSN={2210-271X}, url={http://dx.doi.org/10.1016/J.COMPTC.2014.07.006}, DOI={10.1016/J.COMPTC.2014.07.006}, abstractNote={We have calculated the vertical excitation energy between the singlet ground state (11Ag) and the lowest-lying singlet excited state (11Eu) of Zn-porphyrin employing the fixed-node diffusion Monte Carlo technique. The determinantal parts of trial wave functions were constructed using results from Configuration Interaction and time-dependent Density Functional Theory.}, journal={Computational and Theoretical Chemistry}, publisher={Elsevier BV}, author={Kulahlioglu, A.H. and Mitas, L.}, year={2014}, month={Oct}, pages={6–9} }
@article{rasch_hu_mitas_2014, title={Communication: Fixed-node errors in quantum Monte Carlo: Interplay of electron density and node nonlinearities}, volume={140}, ISSN={0021-9606 1089-7690}, url={http://dx.doi.org/10.1063/1.4862496}, DOI={10.1063/1.4862496}, abstractNote={We elucidate the origin of large differences (two-fold or more) in the fixed-node errors between the first- vs second-row systems for single-configuration trial wave functions in quantum Monte Carlo calculations. This significant difference in the valence fixed-node biases is studied across a set of atoms, molecules, and also Si, C solid crystals. We show that the key features which affect the fixed-node errors are the differences in electron density and the degree of node nonlinearity. The findings reveal how the accuracy of the quantum Monte Carlo varies across a variety of systems, provide new perspectives on the origins of the fixed-node biases in calculations of molecular and condensed systems, and carry implications for pseudopotential constructions for heavy elements.}, number={4}, journal={The Journal of Chemical Physics}, publisher={AIP Publishing}, author={Rasch, Kevin M. and Hu, Shuming and Mitas, Lubos}, year={2014}, month={Jan}, pages={041102} }
@article{dubecky_derian_jurecka_mitas_hobza_otyepka_2014, title={Quantum Monte Carlo for noncovalent interactions: an efficient protocol attaining benchmark accuracy}, volume={16}, ISSN={["1463-9084"]}, DOI={10.1039/c4cp02093f}, abstractNote={A benchmark quantum Monte Carlo protocol with a favorable scaling is reported and extensively tested.}, number={38}, journal={PHYSICAL CHEMISTRY CHEMICAL PHYSICS}, author={Dubecky, Matus and Derian, Rene and Jurecka, Petr and Mitas, Lubos and Hobza, Pavel and Otyepka, Michal}, year={2014}, pages={20915–20923} }
@article{horvathova_derian_mitas_stich_2014, title={Quantum Monte Carlo study of one-dimensional transition-metal organometallic cluster systems and their suitability as spin filters}, volume={90}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.90.115414}, abstractNote={(n = 1 - 3) using advanced quantum Monte Carlo methods. These and relatedsystems have been identi ed as prospective spin lters in spintronic applications, assuming thattheir ground states are half-metallic ferromagnets. Although we nd that magnetic properties ofthese multideckers are consistent with ferromagnetic coupling, their electronic structures do notappear to be half-metallic as previously assumed. In fact, they are ferromagnetic insulators withlarge and broadly similar "-/#-spin gaps. This makes the potential of these and related materialsas spin ltering devices very limited, unless they are further modi ed or functionalized.}, number={11}, journal={PHYSICAL REVIEW B}, author={Horvathova, L. and Derian, R. and Mitas, L. and Stich, I.}, year={2014}, month={Sep} }
@article{kulahlioglu_rasch_hu_mitas_2014, title={Density dependence of fixed-node errors in diffusion quantum Monte Carlo: Triplet pair correlations}, volume={591}, ISSN={["1873-4448"]}, DOI={10.1016/j.cplett.2013.11.033}, abstractNote={Abstract We analyze trial wave function fixed-node errors for a three-electron fully spin-polarized atomic systems with varying atomic number Z . The fully spin-polarized state 4 S ( 1 s 2 s 3 s ) is the lowest quadruplet with S -symmetry and even parity and it is nearly-degenerate with the 4 S ( 1 s 2 p 3 p ) state. We find significant fixed-node errors proportional to Z at the Hartree–Fock level while the two-configuration wave function leads to nearly exact energies. This broadly agrees with our analogous study on Be-like atoms so that the dependence of fixed-node errors on density appears to be similar in both spin-polarized and unpolarized atomic systems.}, journal={CHEMICAL PHYSICS LETTERS}, author={Kulahlioglu, Adem H. and Rasch, Kevin and Hu, Shuming and Mitas, Lubos}, year={2014}, month={Jan}, pages={170–174} }
@article{dubecky_jurecka_derian_hobza_otyepka_mitas_2013, title={Quantum Monte Carlo Methods Describe Noncovalent Interactions with Subchemical Accuracy}, volume={9}, ISSN={["1549-9626"]}, DOI={10.1021/ct4006739}, abstractNote={An accurate description of noncovalent interaction energies is one of the most challenging tasks in computational chemistry. To date, nonempirical CCSD(T)/CBS has been used as a benchmark reference. However, its practical use is limited due to the rapid growth of its computational cost with the system complexity. Here, we show that the fixed-node diffusion Monte Carlo (FN-DMC) method with a more favorable scaling is capable of reaching the CCSD(T)/CBS within subchemical accuracy (<0.1 kcal/mol) on a testing set of six small noncovalent complexes including the water dimer. In benzene/water, benzene/methane, and the T-shape benzene dimer, FN-DMC provides interaction energies that agree within 0.25 kcal/mol with the best available CCSD(T)/CBS estimates. The demonstrated predictive power of FN-DMC therefore provides new opportunities for studies of the vast and important class of medium/large noncovalent complexes.}, number={10}, journal={JOURNAL OF CHEMICAL THEORY AND COMPUTATION}, author={Dubecky, Matus and Jurecka, Petr and Derian, Rene and Hobza, Pavel and Otyepka, Michal and Mitas, Lubos}, year={2013}, month={Oct}, pages={4287–4292} }
@article{zhu_mitas_2013, title={Study of Ne-core and He-core pseudopotential errors in the MnO molecule: Quantum Monte Carlo benchmark}, volume={572}, ISSN={["0009-2614"]}, DOI={10.1016/j.cplett.2013.04.006}, abstractNote={Abstract Accuracy of effective core potential (ECP) is studied for two sizes of cores by density functional theory, Hartree–Fock and quantum Monte Carlo (QMC) methods using the MnO molecule as a test system. We compare the energy differences between high-spin and low-spin states that were previously found to be problematic for transition metal oxide solids calculations with ECPs. In order to disentangle errors caused by ECPs and by subsequent methods used in calculations, we construct a scalar-relativistic He-core ECP for Mn atom. We find that within high quality correlated calculations both Ne-core and He-core ECPs provide energy differences with comparable, high accuracy.}, journal={CHEMICAL PHYSICS LETTERS}, author={Zhu, Minyi and Mitas, Lubos}, year={2013}, month={May}, pages={136–140} }
@article{guo_bajdich_mitas_reynolds_2013, title={Study of dipole moments of LiSr and KRb molecules by quantum Monte Carlo methods}, volume={111}, ISSN={["1362-3028"]}, DOI={10.1080/00268976.2013.788741}, abstractNote={Heteronuclear dimers are of significant interest to experiments seeking to exploit ultracold polar molecules in a number of novel ways, including precision measurement, quantum computing and quantum simulation. We calculate highly accurate Born–Oppenheimer total energies and electric dipole moments as a function of internuclear separation for two such dimers, LiSr and KRb. We apply fully correlated, high-accuracy quantum Monte Carlo methods for evaluating these molecular properties in a many-body framework. We use small-core effective potentials combined with multi-reference Slater–Jastrow trial wave functions to provide accurate nodes for the fixed-node diffusion Monte Carlo method. For reference and comparison, we calculate the same properties with Hartree–Fock and with restricted Configuration Interaction methods, and carefully assess the impact of the recovered many-body correlations on the calculated quantities. For LiSr, we find a highly non-linear dipole moment curve, which may make this molecule’s dipole moment tunable through vibrational state control.}, number={12-13}, journal={MOLECULAR PHYSICS}, author={Guo, Shi and Bajdich, Michal and Mitas, Lubos and Reynolds, Peter J.}, year={2013}, month={Jul}, pages={1744–1752} }
@article{rasch_mitas_2012, title={Impact of electron density on the fixed-node errors in Quantum Monte Carlo of atomic systems}, volume={528}, ISSN={0009-2614}, url={http://dx.doi.org/10.1016/j.cplett.2012.01.016}, DOI={10.1016/j.cplett.2012.01.016}, abstractNote={We analyze the effect of increasing charge density on the fixed-node errors in diffusion Monte Carlo (FNDMC). We compare FNDMC ground state energies of 4-electron atomic systems with nuclear charge Z using Hartree–Fock and restricted Configuration Interaction trial wave functions. We perform these calculations for several different values of the nuclear charge, and we analyze the systematic trends of the recovered correlation energies as a function of Z. For this testing system we demonstrate that the fixed-node error of the Hartree–Fock trial wave functions increases linearly with increasing the nuclear charge.}, journal={Chemical Physics Letters}, publisher={Elsevier BV}, author={Rasch, K.M. and Mitas, L.}, year={2012}, month={Mar}, pages={59–62} }
@inproceedings{hu_rasch_mitas_2012, title={Many-body nodal hypersurface and domain averages for correlated wave functions}, volume={1094}, booktitle={Advances in quantum monte carlo}, author={Hu, S. M. and Rasch, K. and Mitas, L.}, year={2012}, pages={77–87} }
@article{horvathova_dubecky_mitas_stich_2013, title={Quantum Monte Carlo Study of pi-Bonded Transition Metal Organometallics: Neutral and Cationic Vanadium-Benzene and Cobalt-Benzene Half Sandwiches}, volume={9}, ISSN={["1549-9626"]}, DOI={10.1021/ct300887t}, abstractNote={We present accurate quantum Monte Carlo (QMC) calculations that enabled us to determine the structure, spin multiplicity, ionization energy, dissociation energy, and spin-dependent electronic gaps of neutral and positively charged vanadium-benzene and cobalt-benzene systems. From total/ionization energy, we deduce a sextet (quintet) state of neutral (cationic) vanadium-benzene systems and quartet (triplet) state of the neutral (cationic) cobalt-benzene systems. Vastly different energy gaps for the two spin channels are predicted for the vanadium-benzene system and broadly similar energy gaps for the cobalt-benzene system. For this purpose, we have used a multistage combination of techniques with consecutive elimination of systematic biases except for the fixed-node approximation in QMC. Our results significantly differ from the established picture based on previous less accurate calculations and point out the importance of high-level many-body methods for predictive calculations of similar transition metal-based organometallic systems.}, number={1}, journal={JOURNAL OF CHEMICAL THEORY AND COMPUTATION}, author={Horvathova, L. and Dubecky, M. and Mitas, L. and Stich, I.}, year={2013}, month={Jan}, pages={390–400} }
@article{horvathova_dubecky_mitas_stich_2012, title={Spin Multiplicity and Symmetry Breaking in Vanadium-Benzene Complexes}, volume={109}, ISSN={["0031-9007"]}, DOI={10.1103/physrevlett.109.053001}, abstractNote={We present accurate quantum Monte Carlo (QMC) calculations which enabled us to determine the structure, spin multiplicity, ionization energy, dissociation energy, and spin-dependent electronic gaps of the vanadium-benzene system. From total and ionization energy we deduce a high-spin state with vastly different energy gaps for the two spin channels. For this purpose we have used a multistage combination of techniques with consecutive elimination of systematic biases except for the fixed-node approximation in QMC calculations. Our results significantly differ from the established picture based on previous less accurate calculations and point out the importance of high-level many-body methods for predictive calculations of similar transition metal-based organometallic systems.}, number={5}, journal={PHYSICAL REVIEW LETTERS}, author={Horvathova, L. and Dubecky, M. and Mitas, L. and Stich, I.}, year={2012}, month={Jul} }
@article{ambrosetti_silvestrelli_toigo_mitas_pederiva_2012, title={Variational Monte Carlo for spin-orbit interacting systems}, volume={85}, ISSN={["1098-0121"]}, DOI={10.1103/physrevb.85.045115}, abstractNote={Recently, a diffusion Monte Carlo algorithm was applied to the study of spin dependent interactions in condensed matter. Following some of the ideas presented therein, and applied to a Hamiltonian containing a Rashba-like interaction, a general variational Monte Carlo approach is here introduced that treats in an efficient and very accurate way the spin degrees of freedom in atoms when spin orbit effects are included in the Hamiltonian describing the electronic structure. We illustrate the algorithm on the evaluation of the spin-orbit splittings of isolated carbon and lead atoms. In the case of the carbon atom, we investigate the differences between the inclusion of spin-orbit in its realistic and effective spherically symmetrized forms. The method exhibits a very good accuracy in describing the small energy splittings, opening the way for a systematic quantum Monte Carlo studies of spin-orbit effects in atomic systems.}, number={4}, journal={PHYSICAL REVIEW B}, author={Ambrosetti, A. and Silvestrelli, P. L. and Toigo, F. and Mitas, L. and Pederiva, F.}, year={2012}, month={Jan} }
@misc{kolorenc_mitas_2011, title={Applications of quantum Monte Carlo methods in condensed systems}, volume={74}, ISSN={["1361-6633"]}, DOI={10.1088/0034-4885/74/2/026502}, abstractNote={Quantum Monte Carlo methods represent a powerful and broadly applicable computational tool for finding very accurate solutions of the stationary Schrödinger equation for atoms, molecules, solids and a variety of model systems. The algorithms are intrinsically parallel and are able to take full advantage of present-day high-performance computing systems. This review paper concentrates on the fixed-node/fixed-phase diffusion Monte Carlo method with emphasis on its applications to the electronic structure of solids and other extended many-particle systems.}, number={2}, journal={REPORTS ON PROGRESS IN PHYSICS}, author={Kolorenc, Jindrich and Mitas, Lubos}, year={2011}, month={Feb} }
@article{li_kolorenc_mitas_2011, title={Atomic Fermi gas in the unitary limit by quantum Monte Carlo methods: Effects of the interaction range}, volume={84}, ISSN={["1094-1622"]}, DOI={10.1103/physreva.84.023615}, abstractNote={We calculate the ground-state properties of an unpolarized two-component Fermi gas with the aid of the diffusion quantum Monte Carlo (DMC) methods. Using an extrapolation to the zero effective range of the attractive two-particle interaction, we find E/E{sub free} in the unitary limit to be 0.212(2), 0.407(2), 0.409(3), and 0.398(3) for 4, 14, 38, and 66 atoms, respectively. Our calculations indicate that the dependence of the total energy on the effective range of the interaction R{sub eff} is sizable and the extrapolation to R{sub eff}=0 is therefore important for reaching the true unitary limit. To test the quality of nodal surfaces and to estimate the impact of the fixed-node approximation, we perform released-node DMC calculations for 4 and 14 atoms. Analysis of the released-node and the fixed-node results suggests that the main sources of the fixed-node errors are long-range correlations, which are difficult to sample in the released-node approaches due to the fast growth of the bosonic noise. Besides energies, we evaluate the two-body density matrix and the condensate fraction. We find that the condensate fraction for the 66-atom system converges to 0.56(1) after the extrapolation to the zero interaction range.}, number={2}, journal={PHYSICAL REVIEW A}, author={Li, Xin and Kolorenc, Jindrich and Mitas, Lubos}, year={2011}, month={Aug} }
@article{bour_li_lee_meissner_mitas_2011, title={Precision benchmark calculations for four particles at unitarity}, volume={83}, ISSN={["1094-1622"]}, DOI={10.1103/physreva.83.063619}, abstractNote={The unitarity limit describes interacting particles where the range of the interaction is zero and the scattering length is infinite. We present precision benchmark calculations for two-component fermions at unitarity using three different ab initio methods: Hamiltonian lattice formalism using iterated eigenvector methods, Euclidean lattice formalism with auxiliary-field projection Monte Carlo methods, and continuum diffusion Monte Carlo methods with fixed and released nodes. We have calculated the ground-state energy of the unpolarized four-particle system in a periodic cube as a dimensionless fraction of the ground-state energy for the noninteracting system. We obtain values of 0.211(2) and 0.210(2) using two different Hamiltonian lattice representations, 0.206(9) using Euclidean lattice formalism, and an upper bound of 0.212(2) from fixed-node diffusion Monte Carlo methods. Released-node calculations starting from the fixed-node result yield a decrease of less than 0.002 over a propagation of 0.4E{sub F}{sup -1} in Euclidean time, where E{sub F} is the Fermi energy. We find good agreement among all three ab initio methods.}, number={6}, journal={PHYSICAL REVIEW A}, author={Bour, Shahin and Li, Xin and Lee, Dean and Meissner, Ulf-G. and Mitas, Lubos}, year={2011}, month={Jun} }
@article{kolorenc_mitas_2010, title={Electronic structure of solid FeO at high pressures by quantum Monte Carlo methods}, volume={3}, ISSN={["1875-3892"]}, DOI={10.1016/j.phpro.2010.01.203}, abstractNote={Abstract We determine equation of state of stoichiometric FeO by employing the diffusion Monte Carlo method. The fermionic nodes of the many-body wave function are fixed by a single Slater determinant of one-particle orbitals extracted from spin-unrestricted Kohn–Sham equations utilizing a hybrid exchange-correlation functional. The calculated ambient pressure properties agree very well with available experimental data. At approximately 65 GPa, the atomic lattice is found to change from the rocksalt B1 to the NiAs-type inverse B8 structure.}, number={3}, journal={PROCEEDINGS OF THE 22TH WORKSHOP ON COMPUTER SIMULATION STUDIES IN CONDENSED MATTER PHYSICS (CSP 2009)}, author={Kolorenc, Jindrich and Mitas, Lubos}, year={2010}, pages={1437–1441} }
@article{dubecky_derian_mitas_stich_2010, title={Ground and excited electronic states of azobenzene: A quantum Monte Carlo study}, volume={133}, ISSN={["1089-7690"]}, DOI={10.1063/1.3506028}, abstractNote={Large–scale quantum Monte Carlo (QMC) calculations of ground and excited singlet states of both conformers of azobenzene are presented. Remarkable accuracy is achieved by combining medium accuracy quantum chemistry methods with QMC. The results not only reproduce measured values with chemical accuracy but the accuracy is sufficient to identify part of experimental results which appear to be biased. Novel analysis of nodal surface structure yields new insights and control over their convergence, providing boost to the chemical accuracy electronic structure methods of large molecular systems.}, number={24}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Dubecky, M. and Derian, R. and Mitas, L. and Stich, I.}, year={2010}, month={Dec} }
@article{bajdich_kolorenč_mitas_reynolds_2010, title={Pairing in Cold Atoms and other Applications for Quantum Monte Carlo methods}, volume={3}, ISSN={1875-3892}, url={http://dx.doi.org/10.1016/j.phpro.2010.01.199}, DOI={10.1016/j.phpro.2010.01.199}, abstractNote={We discuss the importance of the fermion nodes for the quantum Monte Carlo (QMC) methods and find two cases of the exact nodes. We describe the structure of the generalized pairing wave functions in Pfaffian antisymmetric form and demonstrate their equivalency with certain class of configuration interaction wave functions. We present the QMC calculations of a model fermion system at unitary limit. We find the system to have the energy of E=0.425Efree and the condensate fraction of α=0.48. Further we also perform the QMC calculations of the potential energy surface and the electric dipole moment along that surface of the LiSr molecule. We estimate the vibrationally averaged dipole moment to be 〈D〉ν=0=−0.4(2).}, number={3}, journal={Physics Procedia}, publisher={Elsevier BV}, author={Bajdich, M. and Kolorenč, J. and Mitas, L. and Reynolds, P.J.}, year={2010}, month={Feb}, pages={1397–1410} }
@article{mitas_kolorenc_2010, title={Quantum Monte Carlo Studies of Transition Metal Oxides}, volume={71}, ISBN={["978-0-939950-85-0"]}, ISSN={["1943-2666"]}, DOI={10.2138/rmg.2010.71.7}, abstractNote={Electronic structure of the manganese and iron oxide solids is studied by the quantum Monte Carlo (QMC) methods. The trial wavefunctions are built using orbitals from unrestricted Hartree-Fock and Density Functional Theory, and the electron-electron correlation is recovered by the fixed-node QMC. The correlation effects are significant and QMC estimations of the gap and cohesion show a very good agreement with experiment. Comparison with hybrid functional results points out the importance of the exact exchange for improvement of the Density Functional description of transition metal oxide systems. The QMC methodology have enabled us to calculate the FeO equation of state as well as the transition pressure between the low- and high-pressure FeO structures.}, journal={THEORETICAL AND COMPUTATIONAL METHODS IN MINERAL PHYSICS: GEOPHYSICAL APPLICATIONS}, author={Mitas, Lubos and Kolorenc, Jindrich}, year={2010}, pages={137–145} }
@article{kolorenc_hu_mitas_2010, title={Wave functions for quantum Monte Carlo calculations in solids: Orbitals from density functional theory with hybrid exchange-correlation functionals}, volume={82}, ISSN={["1098-0121"]}, DOI={10.1103/physrevb.82.115108}, abstractNote={We investigate how the fixed-node diffusion Monte Carlo energy of solids depends on single-particle orbitals used in Slater--Jastrow wave functions. We demonstrate that the dependence can be significant, in particular in the case of 3d transition-metal compounds, which we adopt as examples. We illustrate how exchange-correlation functionals with variable exact-exchange component can be exploited to reduce the fixed-node errors. On the basis of these results we argue that the fixed-node quantum Monte Carlo provides a variational approach for optimization of effective hamiltonians with parameters.}, number={11}, journal={PHYSICAL REVIEW B}, author={Kolorenc, Jindrich and Hu, Shuming and Mitas, Lubos}, year={2010}, month={Sep} }
@misc{bajdich_mitas_2009, title={ELECTRONIC STRUCTURE QUANTUM MONTE CARLO}, volume={59}, ISSN={["1336-040X"]}, DOI={10.2478/v10155-010-0095-7}, abstractNote={Electronic structure quantum Monte CarloQuantum Monte Carlo (QMC) is an advanced simulation methodology for studies of manybody quantum systems. The QMC approaches combine analytical insights with stochastic computational techniques for efficient solution of several classes of important many-body problems such as the stationary Schrödinger equation. QMC methods of various flavors have been applied to a great variety of systems spanning continuous and lattice quantum models, molecular and condensed systems, BEC-BCS ultracold condensates, nuclei, etc. In this review, we focus on the electronic structure QMC, i.e., methods relevant for systems described by the electron-ion Hamiltonians. Some of the key QMC achievements include direct treatment of electron correlation, accuracy in predicting energy differences and favorable scaling in the system size. Calculations of atoms, molecules, clusters and solids have demonstrated QMC applicability to real systems with hundreds of electrons while providing 90-95% of the correlation energy and energy differences typically within a few percent of experiments. Advances in accuracy beyond these limits are hampered by the so-called fixed-node approximation which is used to circumvent the notorious fermion sign problem. Many-body nodes of fermion states and their properties have therefore become one of the important topics for further progress in predictive power and efficiency of QMC calculations. Some of our recent results on the wave function nodes and related nodal domain topologies will be briefly reviewed. This includes analysis of few-electron systems and descriptions of exact and approximate nodes using transformations and projections of the highly-dimensional nodal hypersurfaces into the 3D space. Studies of fermion nodes offer new insights into topological properties of eigenstates such as explicit demonstrations that generic fermionic ground states exhibit the minimal number of two nodal domains. Recently proposed trial wave functions based on Pfaffians with pairing orbitals are presented and their nodal properties are tested in calculations of first row atoms and molecules. Finally, backflow "dressed" coordinates are introduced as another possibility for capturing correlation effects and for decreasing the fixed-node bias.}, number={2}, journal={ACTA PHYSICA SLOVACA}, author={Bajdich, Michal and Mitas, Lubos}, year={2009}, pages={81–168} }
@article{wagner_bajdich_mitas_2009, title={QWalk: A quantum Monte Carlo program for electronic structure}, volume={228}, ISSN={["1090-2716"]}, DOI={10.1016/j.jcp.2009.01.017}, abstractNote={We describe QWalk, a new computational package capable of performing quantum Monte Carlo electronic structure calculations for molecules and solids with many electrons. We describe the structure of the program and its implementation of quantum Monte Carlo methods. It is open-source, licensed under the GPL, and available at the web site http://www.qwalk.org.}, number={9}, journal={JOURNAL OF COMPUTATIONAL PHYSICS}, author={Wagner, Lucas K. and Bajdich, Michal and Mitas, Lubos}, year={2009}, month={May}, pages={3390–3404} }
@article{lester_mitas_hammond_2009, title={Quantum Monte Carlo for atoms, molecules and solids}, volume={478}, ISSN={["1873-4448"]}, DOI={10.1016/j.cplett.2009.06.095}, abstractNote={The quantum Monte Carlo (QMC) method has become increasingly important for solution of the stationary Schrödinger equation for atoms, molecules and solids. The method has been shown to exhibit high accuracy that scales better with system size than other ab initio methods. Moreover, as typically implemented, QMC takes full advantage of parallel computing systems. These attributes for electronic structure calculations will be described, as well as recent applications that demonstrate the breadth of the QMC approach.}, number={1-3}, journal={CHEMICAL PHYSICS LETTERS}, author={Lester, William A., Jr. and Mitas, Lubos and Hammond, Brian}, year={2009}, month={Aug}, pages={1–10} }
@article{kino_wagner_mitas_2009, title={Theoretical Study of Electronic and Atomic Structures of (MnO)(n)}, volume={6}, ISSN={["1546-1955"]}, DOI={10.1166/jctn.2009.1318}, abstractNote={We calculate the electronic and atomic structure of (MnO)n (n=1-4) using the HF exchange, VWN, PBE and B3LYP exchange-correlation functionals. We also perform diffusion Monte Carlo calculation to evaluate more accurate energies. We ompare these results and discuss the accuracy of the exchange-correlation functionals.}, number={12}, journal={JOURNAL OF COMPUTATIONAL AND THEORETICAL NANOSCIENCE}, author={Kino, Hiori and Wagner, Lucas K. and Mitas, Lubos}, year={2009}, month={Dec}, pages={2583–2588} }
@article{krcmar_gendiar_mosko_nemeth_vagner_mitas_2008, title={Persistent current of correlated electrons in mesoscopic ring with impurity}, volume={40}, ISSN={["1873-1759"]}, DOI={10.1016/j.physe.2007.09.074}, abstractNote={The persistent current of correlated electrons in a continuous one-dimensional ring with a single scatterer is calculated by solving the many-body Schrodinger equation for several tens of electrons interacting via the electron–electron (e–e) interaction of finite range. The problem is solved by the configuration-interaction (CI) and diffusion Monte Carlo (DMC) methods. The CI and DMC results are in good agreement. In both cases, the persistent current I as a function of the ring length L exhibits the asymptotic dependence I∝L-1-α typical of the Luttinger liquid, where the power α depends only on the e–e interaction. The numerical values of α agree with the known formula of the renormalization-group theory.}, number={5}, journal={PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES}, author={Krcmar, R. and Gendiar, A. and Mosko, M. and Nemeth, R. and Vagner, P. and Mitas, L.}, year={2008}, month={Mar}, pages={1507–1509} }
@article{bajdich_mitas_wagner_schmidt_2008, title={Pfaffian pairing and backflow wavefunctions for electronic structure quantum Monte Carlo methods}, volume={77}, ISSN={["1098-0121"]}, DOI={10.1103/physrevb.77.115112}, abstractNote={We investigate pfaffian trial wavefunctions with singlet and triplet pair orbitals by quantum Monte Carlo methods. We present mathematical identities and the key algebraic properties necessary for efficient evaluation of pfaffians. Following upon our previous study [Bajdich et al., Phys. Rev. Lett. 96, 130201 (2006)], we explore the possibilities of expanding the wavefunction in linear combinations of pfaffians. We observe that molecular systems require much larger expansions than atomic systems and linear combinations of a few pfaffians lead to rather small gains in correlation energy. We also test the wavefunction based on fully antisymmetrized product of independent pair orbitals. Despite its seemingly large variational potential, we do not observe additional gains in correlation energy. We find that pfaffians lead to substantial improvements in fermion nodes when compared to Hartree-Fock wavefunctions and exhibit the minimal number of two nodal domains in agreement with recent results on fermion nodes topology. We analyze the nodal structure differences of Hartree-Fock, pfaffian, and essentially exact large-scale configuration interaction wavefunctions. Finally, we combine the recently proposed form of backflow correlations [Drummond et al., J. Phys. Chem. 124, 22401 (2006); Rios et al., Phys. Rev. E. 74, 066701 (2006)] with both determinantal and pfaffian based wavefunctions.}, number={11}, journal={PHYSICAL REVIEW B}, author={Bajdich, M. and Mitas, L. and Wagner, L. K. and Schmidt, K. E.}, year={2008}, month={Mar} }
@article{kolorenc_mitas_2007, title={B1-to-B8 structural phase transition in MnO under pressure: Comparison of all-electron and pseudopotential approaches}, volume={75}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.75.235118}, abstractNote={We employ the density-functional theory to study a structural transition of MnO from B1 rocksalt to B8 NiAs structures that was observed experimentally at pressures around 100 GPa. We utilize all-electron description as well as norm-conserving pseudopotentials and demonstrate that these two approaches can significantly differ in quantitative predictions. We explicitly show that even small-core pseudopotentials exhibit transferability inaccuracies for quantities sensitive to the energy differences between high- and low-spin polarizations of valence electrons.}, number={23}, journal={PHYSICAL REVIEW B}, author={Kolorenc, Jindrich and Mitas, Lubos}, year={2007}, month={Jun} }
@article{wagner_mitas_2007, title={Energetics and dipole moment of transition metal monoxides by quantum Monte Carlo}, volume={126}, ISSN={["1089-7690"]}, DOI={10.1063/1.2428294}, abstractNote={The transition metal (TM) oxygen bond appears very prominently throughout chemistry and solid-state physics. Many materials, from biomolecules to ferroelectrics to the components of supernova remnants, contain this bond in some form. Many of these materials’ properties depend strongly on fine details of the TM–O bond, which makes accurate calculations of their properties very challenging. Here the authors report on highly accurate first principles calculations of the properties of TM monoxide molecules within fixed-node diffusion Monte Carlo and reptation Monte Carlo.}, number={3}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Wagner, Lucas K. and Mitas, Lubos}, year={2007}, month={Jan} }
@article{vagner_moško_németh_wagner_mitas_2006, title={Hartree–Fock versus quantum Monte Carlo study of persistent current in a one-dimensional ring with single scatterer}, volume={32}, ISSN={1386-9477}, url={http://dx.doi.org/10.1016/j.physe.2005.12.062}, DOI={10.1016/j.physe.2005.12.062}, abstractNote={We calculate the persistent current of interacting spinless electrons in a one-dimensional ring containing a single δ barrier. We use the self-consistent Hartree–Fock method and the quantum Monte Carlo method which gives fully correlated solutions. Our Hartree–Fock method treats the non-local Fock term in a local approximation and also exactly (if the ring is not too large). Treating the Fock term exactly we attempt to support our previous Hartree–Fock result obtained in the local approximation, in particular the persistent current behaving like I∝L-1-α, where L is the ring length and α>0 is the power depending only on the electron–electron interaction. Finally, we use the Hartree–Fock solutions as an input for our quantum Monte Carlo calculation. The Monte Carlo results exhibit only small quantitative differences from the Hartree–Fock results.}, number={1-2}, journal={Physica E: Low-dimensional Systems and Nanostructures}, publisher={Elsevier BV}, author={Vagner, Pavel and Moško, Martin and Németh, Radoslav and Wagner, Lucas and Mitas, Lubos}, year={2006}, month={May}, pages={350–353} }
@article{bajdich_mitas_drobny_wagner_schmidt_2006, title={Pfaffian pairing wave functions in electronic-structure quantum Monte Carlo simulations}, volume={96}, ISSN={["1079-7114"]}, DOI={10.1103/physrevlett.96.130201}, abstractNote={We investigate the accuracy of trial wave functions for quantum Monte Carlo based on Pfaffian functional form with singlet and triplet pairing. Using a set of first row atoms and molecules we find that these wave functions provide very consistent and systematic behavior in recovering the correlation energies on the level of 95%. In order to get beyond this limit we explore the possibilities of multi-Pfaffian pairing wave functions. We show that a small number of Pfaffians recovers another large fraction of the missing correlation energy comparable to the larger-scale configuration interaction wave functions. We also find that Pfaffians lead to substantial improvements in fermion nodes when compared to Hartree-Fock wave functions.}, number={13}, journal={PHYSICAL REVIEW LETTERS}, author={Bajdich, M and Mitas, L and Drobny, G and Wagner, LK and Schmidt, KE}, year={2006}, month={Apr} }
@article{mitasova_mitas_ratti_ishii_alonso_harmon_2006, title={Real-time landscape model interaction using a tangible geospatial modeling environment}, volume={26}, ISSN={["1558-1756"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33746079196&partnerID=MN8TOARS}, DOI={10.1109/MCG.2006.87}, abstractNote={Emerging technologies that combine the flexibility of digital landscape representation with easy-to-interpret 3D physical models open new possibilities for user interaction with geospatial data. A prototype tangible geospatial modeling environment lets users interact with landscape analysis and simulations using a tangible physical model. We introduce a concept that builds upon previous independent tangible user interface (TUI) and terrain analysis research and aims at more intuitive collaborative interaction with digital landscape data.}, number={4}, journal={IEEE COMPUTER GRAPHICS AND APPLICATIONS}, author={Mitasova, Helena and Mitas, Lubos and Ratti, Carlo and Ishii, Hiroshi and Alonso, Jason and Harmon, Russell S.}, year={2006}, pages={55–63} }
@article{mitas_2006, title={Structure of fermion nodes and nodal cells}, volume={96}, ISSN={["1079-7114"]}, DOI={10.1103/physrevlett.96.240402}, abstractNote={We study nodes of fermionic ground state wave functions. For two dimensions and higher we prove that spin-polarized, noninteracting fermions in a harmonic well have two nodal cells for arbitrary system size. The result extends to noninteracting or mean-field models in other geometries and to Hartree-Fock atomic states. Spin-unpolarized noninteracting states have multiple nodal cells; however, interactions and many-body correlations generally relax the multiple cells to the minimal number of two. With some conditions, this is proved for interacting two and higher dimensions harmonic fermion systems of arbitrary size using the Bardeen-Cooper-Schrieffer variational wave function.}, number={24}, journal={PHYSICAL REVIEW LETTERS}, author={Mitas, Lubos}, year={2006}, month={Jun} }
@article{bajdich_mitas_drobny_wagner_2005, title={Approximate and exact nodes of fermionic wavefunctions: Coordinate transformations and topologies}, volume={72}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.72.075131}, abstractNote={A study of fermion nodes for spin-polarized states of a few-electron ions and molecules with s,p,d one-particle orbitals is presented. We find exact nodes for some cases of two-electron atomic and molecular states and also the first exact node for the three-electron atomic system in {sup 4}S(p{sup 3}) state using appropriate coordinate maps and wave function symmetries. We analyze the cases of nodes for larger number of electrons in the Hartree-Fock approximation and for some cases we find transformations for projecting the high-dimensional node manifolds into three-dimensional space. The node topologies and other properties are studied using these projections. We also propose a general coordinate transformation as an extension of Feynman-Cohen backflow coordinates to both simplify the nodal description and as a new variational freedom for quantum Monte Carlo trial wave functions.}, number={7}, journal={PHYSICAL REVIEW B}, author={Bajdich, M and Mitas, L and Drobny, G and Wagner, LK}, year={2005}, month={Aug} }
@article{grossman_mitas_2005, title={Efficient quantum monte carlo energies for molecular dynamics simulations}, volume={94}, ISSN={["1079-7114"]}, DOI={10.1103/physrevlett.94.056403}, abstractNote={A method is presented to treat electrons within the many-body quantum Monte Carlo (QMC) approach "on-the-fly" throughout a molecular dynamics (MD) simulation. Our approach leverages the large (10-100) ratio of the QMC electron to MD ion motion to couple the stochastic, imaginary-time electronic and real-time ionic trajectories. This continuous evolution of the QMC electrons results in highly accurate total energies for the full dynamical trajectory at a fraction of the cost of conventional, discrete sampling. We show that this can be achieved efficiently for both ground and excited states with only a modest overhead to an ab initio MD method. The accuracy of this dynamical QMC approach is demonstrated for a variety of systems, phases, and properties, including optical gaps of hot silicon quantum dots, dissociation energy of a single water molecule, and heat of vaporization of liquid water.}, number={5}, journal={PHYSICAL REVIEW LETTERS}, author={Grossman, JC and Mitas, L}, year={2005}, month={Feb} }
@article{mitasova_mitas_harmon_2005, title={Simultaneous spline approximation and topographic analysis for lidar elevation data in open-source GIS}, volume={2}, ISSN={["1558-0571"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-27744590597&partnerID=MN8TOARS}, DOI={10.1109/LGRS.2005.848533}, abstractNote={Application of a spline approximation method to computation and analysis of lidar-based digital elevation models is investigated to determine its accuracy and capability to create surfaces at different levels of detail. Quadtree segmentation that adapts to the spatial heterogeneity of data points makes the method feasible for large datasets. The results demonstrate the importance of smoothing for the surface accuracy and noise reduction. A tension parameter is effective for tuning the level of detail in the elevation surface. Simultaneous computation of topographic parameters is applied to extraction of sand dunes' features for assessment of dune migration and beach erosion.}, number={4}, journal={IEEE GEOSCIENCE AND REMOTE SENSING LETTERS}, author={Mitasova, H and Mitas, L and Harmon, RS}, year={2005}, month={Oct}, pages={375–379} }
@article{harkless_rodriguez_mitas_lester_2003, title={A quantum Monte Carlo and density functional theory study of the electronic structure of peroxynitrite anion}, volume={118}, ISSN={["0021-9606"]}, DOI={10.1063/1.1544732}, abstractNote={Single point calculations of the ground state electronic structure of peroxynitrite anion have been performed at the optimized cis geometry using the restricted Hartree–Fock (RHF), Møller Plesset second order perturbation theory (MP2), generalized gradient approximation density functional theory (GGA DFT) in the B3LYP form and two quantum Monte Carlo (QMC) methods, variational Monte Carlo (VMC) and diffusion Monte Carlo (DMC). These calculations reveal differences in atomization energies estimated by B3LYP (287.03 kcal/mol), MP2 (290.84 kcal/mol), and DMC, 307.4(1.9) kcal/mol, as compared to experiment, 313(1) kcal/mol. The error associated with MP2 and B3LYP methods is attributed largely to differential recovery of correlation energies for neutral nitrogen and oxygen atoms relative to the oxygen and peroxynitrite anions. In addition, basis set studies were carried out to determine potential sources of error in MP2 and B3LYP valence energy values. Our studies indicate that MP2 and B3LYP valence energies are strongly dependent on the presence of diffuse functions for the negative ions O− and ONOO−.}, number={11}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Harkless, JAW and Rodriguez, JH and Mitas, L and Lester, WA}, year={2003}, month={Mar}, pages={4987–4992} }
@article{wagner_mitas_2003, title={A quantum Monte Carlo study of electron correlation in transition metal oxygen molecules}, volume={370}, ISSN={["1873-4448"]}, DOI={10.1016/S0009-2614(03)00128-3}, abstractNote={We carry out calculations of selected transition metal–oxygen molecules within density functional theory, post-Hartree–Fock, and quantum Monte Carlo (QMC) methods. Transition metal–oxygen systems have competing electron correlation and exchange effects and require accurate treatment of both of these effects. We analyze the biases of the mentioned methods and their impacts on the electronic structure. We evaluate binding energies and compare the accuracy of various approaches including single and multi-reference trial wave functions in QMC.}, number={3-4}, journal={CHEMICAL PHYSICS LETTERS}, author={Wagner, L and Mitas, L}, year={2003}, month={Mar}, pages={412–417} }
@article{sen_mitas_2003, title={Electronic structure and ground states of transition metals encapsulated in a Si-12 hexagonal prism cage}, volume={68}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.68.155404}, abstractNote={We report on a computational study of the electronic structure of recently discovered clusters with an encapsulated transition metal (TM) atom in a ${\mathrm{Si}}_{12}$ hexagonal prism cage. The cage geometry is remarkably stable regardless of the type of doping TM atom from $3d,4d,$ and $5d$ series. We predict and quantify the stability for several other TM dopings besides the experimentally observed ones. The multiplicity of the ground states can be ``tuned'' between singlets and triplets by varying the type of TM atom (even number of electrons), while they are doublets for odd number of electrons. We also explore the possibility of forming solids with hexagonal structure from selected clusters.}, number={15}, journal={PHYSICAL REVIEW B}, author={Sen, P and Mitas, L}, year={2003}, month={Oct} }
@article{belomoin_rogozhina_therrien_braun_abuhassan_nayfeh_wagner_mitas_2002, title={Effects of surface termination on the band gap of ultrabright Si-29 nanoparticles: Experiments and computational models}, volume={65}, ISSN={["1098-0121"]}, DOI={10.1103/physrevb.65.193406}, abstractNote={A Si 2 9 H 2 4 particle, with five atoms constituting a tetrahedral core and 24 atoms constituting a H-terminated reconstructed Si surface was recently proposed as a structural prototype of ultrasmall ultrabright particlesprepared by electrochemical dispersion from bulk Si. We replace the H termination with a N linkage (in butylamine) and O linkage (in pentane). The emission band for N-termination downshifts by ∼0.25 eV from that of H termination, whereas it blueshift∼0.070 eV for C termination. We use density-functional approaches to calculate the atomic structures and correction from the quantum Monte Carlo method to estimate the highest occupied-lowest unoccupied molecular-orbital band gap. We find a downshift of 0.25 eV for N termination and very little for C termination. These features are discussed in terms of exciton penetration in the capping material.}, number={19}, journal={PHYSICAL REVIEW B}, author={Belomoin, G and Rogozhina, E and Therrien, J and Braun, PV and Abuhassan, L and Nayfeh, MH and Wagner, L and Mitas, L}, year={2002}, month={May} }
@article{bokes_stich_mitas_2002, title={Ground-state reconstruction of the Si(001) surface: symmetric versus buckled dimers}, volume={362}, ISSN={["1873-4448"]}, DOI={10.1016/S0009-2614(02)01081-3}, abstractNote={An extensive computational study is presented with the quest to investigate the nature of the ground-state geometry of the Si(0 0 1) surface, a subject of recent experimental controversy. We analyze for the first time in detail the possible sources of errors which would arise in any correlated calculation for a system size of interest here. For this purpose, we present a detailed analysis of the cluster model of the surface at the DFT and MCSCF level of theory. Estimates of errors arising from the use of pseudopotential, finite cluster size, and biased (method dependent) choice of ground-state geometry are given. The resulting error is estimated to be comparable to the energy scale of interest. On the other hand, the energy variation due to negative thermal expansion at low temperature is found to be qualitatively consistent with dimer symmetrization.}, number={5-6}, journal={CHEMICAL PHYSICS LETTERS}, author={Bokes, P and Stich, I and Mitas, L}, year={2002}, month={Aug}, pages={559–566} }
@article{belomoin_therrien_smith_rao_twesten_chaieb_nayfeh_wagner_mitas_2002, title={Observation of a magic discrete family of ultrabright Si nanoparticles}, volume={80}, ISSN={["0003-6951"]}, DOI={10.1063/1.1435802}, abstractNote={We demonstrate that electrochemically etched, hydrogen capped SinHx clusters with n larger than 20 are obtained within a family of discrete sizes. These sizes are 1.0 (Si29), 1.67 (Si123), 2.15, 2.9, and 3.7 nm in diameter. We characterize the particles via direct electron imaging, excitation and emission optical spectroscopy, and colloidal crystallization. The band gaps and emission bands are measured. The smallest four are ultrabright blue, green, yellow and red luminescent particles. The availability of discrete sizes and distinct emission in the red, green and blue (RGB) range is useful for biomedical tagging, RGB displays, and flash memories.}, number={5}, journal={APPLIED PHYSICS LETTERS}, author={Belomoin, G and Therrien, J and Smith, A and Rao, S and Twesten, R and Chaieb, S and Nayfeh, MH and Wagner, L and Mitas, L}, year={2002}, month={Feb}, pages={841–843} }
@article{mitas_2002, title={Quantum Monte Carlo methods for electronic structure of nanosystems}, volume={42}, ISSN={["0021-2148"]}, DOI={10.1560/QRWB-75NV-MEL1-D124}, abstractNote={AbstractWe provide a brief review of recent applications of quantum Monte Carlo (QMC) methods to the electronic structure of nanosystems. We report on calculations of carbon rings with second‐order Jahn‐Teller effect, energy ordering of silicon clusters, dissociation enthalpies of protonated hydrogen clusters, and other interesting challenges. We point out the QMC accuracy and outline a few ideas that characterize the current position of QMC among the electronic structure methods and its future development.}, number={2-3}, journal={ISRAEL JOURNAL OF CHEMISTRY}, author={Mitas, L}, year={2002}, pages={261–268} }
@article{mitas_therrien_twesten_belomoin_nayfeh_2001, title={Effect of surface reconstruction on the structural prototypes of ultrasmall ultrabright Si-29 nanoparticles}, volume={78}, ISSN={["0003-6951"]}, DOI={10.1063/1.1356447}, abstractNote={We propose, using density functional, configuration interaction, and quantum Monte Carlo calculations, structural prototypes of ultrasmall ultrabright particles prepared by dispersion from bulk. We constructed near spherical structures (Td point group symmetry) that contain 29 Si atoms, five of which constitute a tetrahedral core and the remaining 24 constitute a hydrogen terminated reconstructed Si surface. The surface is a highly wrinkled or puckered system of hexagons and pentagons (as in a filled fullerene). We calculated, for several surface reconstruction models, the coordinates of atoms, the absorption spectrum, the absorption edge, polarizability, and the electron diffraction pattern. The Si29H24 (six reconstructed surface dimers) gives a size of 0.9 nm, an absorption spectrum and bandgap (3.5±0.3 eV), in fair agreement with measurement. The structure yields a polarizability of 830 a.u. with an effective “dielectric” constant of ∼6.0. The calculated electron diffraction of single particles shows residual crystalline coherent scattering for large but not small scattering angles.}, number={13}, journal={APPLIED PHYSICS LETTERS}, author={Mitas, L and Therrien, J and Twesten, R and Belomoin, G and Nayfeh, MH}, year={2001}, month={Mar}, pages={1918–1920} }
@misc{foulkes_mitas_needs_rajagopal_2001, title={Quantum Monte Carlo simulations of solids}, volume={73}, ISSN={["1539-0756"]}, DOI={10.1103/revmodphys.73.33}, abstractNote={This article describes the variational and fixed-node diffusion quantum Monte Carlo methods and how they may be used to calculate the properties of many-electron systems. These stochastic wave-function-based approaches provide a very direct treatment of quantum many-body effects and serve as benchmarks against which other techniques may be compared. They complement the less demanding density-functional approach by providing more accurate results and a deeper understanding of the physics of electronic correlation in real materials. The algorithms are intrinsically parallel, and currently available high-performance computers allow applications to systems containing a thousand or more electrons. With these tools one can study complicated problems such as the properties of surfaces and defects, while including electron correlation effects with high precision. The authors provide a pedagogical overview of the techniques and describe a selection of applications to ground and excited states of solids and clusters.}, number={1}, journal={REVIEWS OF MODERN PHYSICS}, author={Foulkes, WMC and Mitas, L and Needs, RJ and Rajagopal, G}, year={2001}, month={Jan}, pages={33–83} }