Variation After Response in Quantum Monte Carlo

TL;DR

This paper introduces a variational method for modeling electronically excited states that allows the ground state to relax during excitation calculations, improving accuracy especially for doubly excited states.

Contribution

It presents a novel variation-after-response approach that overcomes limitations of linear response theory in excited state modeling, with comparable or superior accuracy to high-level methods.

Findings

Achieves accuracy comparable to equation of motion coupled cluster for valence and charge transfer excitations.

Surpasses EOM-CC in accuracy for excitations with significant double excitation character.

Method is variational, cost-effective, and compatible with open and periodic boundary conditions.

Abstract

We present a new method for modeling electronically excited states that overcomes a key failing of linear response theory by allowing the underlying ground state ansatz to relax in the presence of an excitation. The method is variational, has a cost similar to ground state variational Monte Carlo, and admits both open and periodic boundary conditions. We present preliminary numerical results showing that, when paired with the Jastrow antisymmetric geminal power ansatz, the variation-after-response formalism delivers accuracies for valence and charge transfer single excitations on par with equation of motion coupled cluster, while surpassing even this very high-level method's accuracy for excitations with significant doubly excited character.