@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{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{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_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_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} }