@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{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{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} }
@misc{wagner_2007, title={Transition metal oxides using quantum Monte Carlo}, volume={19}, number={34}, journal={Journal of Physics. Condensed Matter}, author={Wagner, L. K.}, year={2007} }
@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{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{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{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} }