@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{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{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{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} }
 @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} }
 @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{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{froehlich_casanova_hempel_liebendoerfer_melton_perego_2014, title={Neutrinos and nucleosynthesis in core-collapse supernovae}, volume={1604}, ISSN={["0094-243X"]}, DOI={10.1063/1.4883428}, abstractNote={Massive stars (M > 8-10 M⊙) undergo core collapse at the end of their life and explode as supernova with ∼ 1051 erg of kinetic energy. While the detailed supernova explosion mechanism is still under investigation, reliable nucleosynthesis calculations based on successful explosions are needed to explain the observed abundances in metal-poor stars and to predict supernova yields for galactic chemical evolution studies. To predict nucleosynthesis yields for a large number of progenitor stars, computationally efficient explosion models are required. We model the core collapse, bounce and subsequent explosion of massive stars assuming spherical symmetry and using detailed microphysics and neutrino physics combined with a novel method to artificially trigger the explosion (PUSH). We discuss the role of neutrinos, the conditions in the ejecta, and the resulting nucleosynthesis.}, journal={WORKSHOP ON DARK MATTER, NEUTRINO PHYSICS AND ASTROPHYSICS CETUP 2013: VIITH INTERNATIONAL CONFERENCE ON INTERCONNECTIONS BETWEEN PARTICLE PHYSICS AND COSMOLOGY PPC 2013}, author={Froehlich, C. and Casanova, J. and Hempel, M. and Liebendoerfer, M. and Melton, C. A. and Perego, A.}, year={2014}, pages={178–184} }