Double excitation energies from quantum Monte Carlo using state-specific energy optimization

TL;DR

This paper demonstrates that advanced quantum Monte Carlo methods can accurately compute double excitation energies in molecules, including challenging cases where traditional high-level methods struggle, offering reliable benchmarks and predictions.

Contribution

It introduces the application of state-specific energy optimization in quantum Monte Carlo to effectively treat double excitations in molecules.

Findings

QMC methods yield accurate double excitation energies.

Results agree well with benchmarks where available.

Provides predictions for systems lacking reference data.

Abstract

We show that recently developed quantum Monte Carlo methods, which provide accurate vertical transition energies for single excitations, also successfully treat double excitations. We study the double excitations in medium-sized molecules, some of which are challenging for high level coupled-cluster calculations to model accurately. Our fixed-node diffusion Monte Carlo excitation energies are in very good agreement with reliable benchmarks, when available, and provide accurate predictions for excitation energies of difficult systems where reference values are lacking.