Excited States with Selected Configuration Interaction-Quantum MonteCarlo: Chemically Accurate Excitation Energies and Geometries

摘要

We employ quantum Monte Carlo to obtain chemically accurate vertical and adiabatic excitation energies, and equilibrium excited-state structures for the small, yet challenging, formaldehyde and thioformaldehyde molecules. A key ingredient is a robust protocol to obtain balanced ground- and excited-state Jastrow–Slater wave functions at a given geometry, and to maintain such a balanced description as we relax the structure in the excited state. We use determinantal components generated via a selected configuration interaction scheme which targets the same second-order perturbation energy correction for all states of interest at different geometries, and fully optimize all variational parameters in the resultant Jastrow–Slater wave functions. Importantly, the excitation energies as well as the structural parameters in the ground and excited states are converged with very compact wave functions comprising few thousand determinants in a minimally augmented double-ζ basis set. These results are obtained already at the variational Monte Carlo level, the more accurate diffusion Monte Carlomethod yielding only a small improvement in the adiabatic excitationenergies. We find that matching Jastrow–Slater wave functionswith similar variances can yield excitation energies compatible withour best estimates; however, the variance-matching procedure requiressomewhat larger determinantal expansions to achieve the same accuracy,and it is less straightforward to adapt during structural optimizationin the excited state.