@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_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_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{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} }