# Excited states with selected CI-QMC: chemically accurate excitation energies and geometries

## TL;DR

This paper demonstrates that quantum Monte Carlo methods with selected CI-generated wave functions can achieve chemically accurate excitation energies and geometries for formaldehyde and thioformaldehyde, using compact wave functions and a robust protocol.

## Contribution

It introduces a robust protocol for balanced ground- and excited-state wave functions and applies selected CI with variational Monte Carlo to obtain accurate excited-state properties.

## Key findings

- Chemically accurate excitation energies and geometries achieved.
- Compact wave functions with few thousand determinants are sufficient.
- Diffusion Monte Carlo provides minor improvements over variational results.

## Abstract

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 we 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-$\zeta$ basis set. These results are obtained already at the variational Monte Carlo level, the more accurate diffusion Monte Carlo method yielding only a small improvement in the adiabatic excitation energies. We find that matching Jastrow-Slater wave functions with similar variances can yield excitations compatible with our best estimates; however, the variance-matching procedure requires somewhat larger determinantal expansions to achieve the same accuracy, and it is less straightforward to adapt during structural optimization in the excited state.