Excited-state diffusion Monte Carlo calculations: a simple and efficient two-determinant ansatz

TL;DR

This paper introduces a simple two-determinant wave function ansatz for excited-state diffusion Monte Carlo calculations, improving accuracy and efficiency by incorporating orbital relaxation and comparing results with near-exact quantum methods.

Contribution

The authors develop a novel two-determinant ansatz for excited states that enhances diffusion Monte Carlo accuracy and efficiency, with detailed orbital optimization procedures.

Findings

Accurate excited-state energies for water and formaldehyde.

Orbital optimization significantly improves DMC results.

The ansatz performs well with large basis sets.

Abstract

We perform excited-state variational Monte Carlo and diffusion Monte Carlo calculations using a simple and efficient wave function ansatz. This ansatz follows the recent variation-after-response formalism, accurately approximating a configuration interaction singles wave function as a sum of only two non-orthogonal Slater determinants, and further including important orbital relaxation. The ansatz is used to perform diffusion Monte Carlo calculations with large augmented basis sets, comparing to benchmarks from near-exact quantum chemical methods. The significance of orbital optimization in excited-state diffusion Monte Carlo is demonstrated, and the excited-state optimization procedure is discussed in detail. Diffuse excited states in water and formaldehyde are studied, in addition to the formaldimine and benzonitrile molecules.