@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{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 (BeH$_2$) and acetylene (C$_2$H$_2$) 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{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 $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ 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 $\mathrm{DFT}+U$ 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, $\text{DFT}+U$, 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_hu_wang_heinonen_kent_krogel_ganesh_2022, title={Origin of metal-insulator transitions in correlated perovskite metals}, volume={4}, ISSN={["2643-1564"]}, DOI={10.1103/PhysRevResearch.4.L022005}, abstractNote={The mechanisms that drive metal-to-insulator transitions (MIT) in correlated solids are not fully understood, though intricate couplings of charge, spin, orbital, and lattice degrees of freedom have been implicated. For example, the perovskite SrCoO3 is a ferromagnetic metal, while the oxygen-deficient (n-doped) brownmillerite SrCoO2.5 is an antiferromagnetic insulator. Given the magnetic and structural transitions that accompany the MIT, the driving force for such a MIT transition is unclear. We also observe that, interestingly, the perovskite metals LaNiO3, SrFeO3, and SrCoO3 also undergo MIT when n-doped via high-to-low valence compositional changes, i.e., Ni3+→Fe4+, Sr2+→La3+, and Sr2+→La3+, respectively. On the other hand, pressurizing the insulating brownmillerite SrCoO2.5 phase drives a gap closing. Here we demonstrate that the ABO3 perovskites most prone to MIT are self-hole-doped materials, reminiscent of a negative charge-transfer metal, using a combination of density functional and fixed-node diffusion quantum Monte Carlo calculations. Upon n doping the negative charge-transfer metallic phase, an underlying charge-lattice (or electron-phonon) coupling drives the metal to a charge and bond-disproportionated gapped insulating state, thereby achieving ligand-hole passivation at certain sites only. The size of the band gap is linearly correlated with the degree of hole passivation at these ligand sites. Further, metallization via pressure is also stabilized by a similar increase in the ligand hole, which in turn stabilizes the ferromagnetic coupling. These results suggest that the interaction that drives the band-gap opening to realize a MIT even in correlated metals is the charge-transfer energy, while it couples with the underlying phonons to enable the transition to the insulating phase. Other orderings (magnetic, charge, orbital etc.) driven by weaker interactions may assist gap openings at low doping levels, but it is the charge-transfer energy that predominantly determines the band gap, with a negative energy preferring the metallic phase. This n doping can be achieved by modulations in oxygen stoichiometry or metal composition or pressure. Hence, controlling the amount of the ligand hole, set by the charge-transfer energy, is the key factor in controlling MIT.Received 16 July 2021Accepted 24 January 2022DOI:https://doi.org/10.1103/PhysRevResearch.4.L022005Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasMetal-insulator transitionPhysical SystemsPerovskitesTransition metal oxidesTechniquesDensity functional theoryDiffusion quantum Monte CarloElectron-correlation calculationsCondensed Matter, Materials & Applied Physics}, number={2}, journal={PHYSICAL REVIEW RESEARCH}, author={Bennett, M. Chandler and Hu, Guoxiang and Wang, Guangming and Heinonen, Olle and Kent, Paul R. C. and Krogel, Jaron T. and Ganesh, P.}, year={2022}, month={Apr} }
 @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 $\textit{systematic and statistical errors}$ below $\approx$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 $\approx$ 1-10 mHa for K-Zn atoms and $\approx$ 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 Si$_x$H$_y$ molecular systems both in 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 $\approx$ 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 $1-3.5$ % with absolute values at most $\approx$ 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{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={Abstract}, 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 3$^{rd}$-row main group elements, creating new correlation consistent effective core potentials (ccECPs) and also derive 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]$3d^{10}$-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{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} }
 @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} }