Valence and core ionization energies of atoms and small molecules using variational Monte Carlo with explicit correlation and Slater-type basis sets.

contextReliable prediction of both valence and core ionization energies remains challenging because these processes involve different balances of electron correlation, orbital relaxation, and basis-set flexibility. In this work, we assess whether variational Monte Carlo (VMC) with explicit correlation and compact Slater-type basis sets can provide accurate ionization energies without fitting to experimental data. As expected from their greater flexibility, double-zeta (DZ) and Hartree-Fock limit (HFL) Slater-type basis sets provide better agreement with experiment than single-zeta (SZ), particularly for atomic ionizations and some molecular cases. For H

methodsHartree-Fock wave functions with Slater-type basis sets were used as trial wave functions. For atoms, the SZ, DZ, and HFL orbital expansions were taken from the literature. For neutral molecules, Hartree-Fock calculations were performed with Gaussian 09 using Slater-type basis sets from the literature to generate the corresponding LCAO coefficients. Single valence and core ionization energies were computed with the VMC method using a nine-parameter Boys-Handy explicit correlation factor and Slater-type basis sets, with and without polarization functions. The VMC calculations and parameter optimizations were performed with computer programs developed by the authors' research group.