An efficient and accurate perturbative correction to initiator full configuration interaction quantum Monte Carlo

TL;DR

This paper introduces a perturbative correction to the initiator FCIQMC method, enabling more accurate energy calculations by utilizing discarded walkers to compute a second-order Epstein-Nesbet correction efficiently.

Contribution

The paper presents a novel perturbative correction method that improves the accuracy of i-FCIQMC by leveraging discarded walkers for a second-order correction.

Findings

Significant improvement in i-FCIQMC results with the correction

Effective application to Hubbard model, electron gas, and molecules

Correction is computationally inexpensive and easy to implement

Abstract

We present a perturbative correction within initiator full configuration interaction quantum Monte Carlo (i-FCIQMC). In the existing i-FCIQMC algorithm, a significant number of spawned walkers are discarded due to the initiator criteria. Here we show that these discarded walkers have a form that allows calculation of a second-order Epstein-Nesbet correction, that may be accumulated in a trivial and inexpensive manner, yet substantially improves i-FCIQMC results. The correction is applied to the Hubbard model, the uniform electron gas and molecular systems.