Response Formalism within Full Configuration Interaction Quantum Monte Carlo: Static Properties and Electrical Response

TL;DR

This paper introduces a stochastic response formalism within FCIQMC that enables accurate computation of response properties for complex multireference systems, demonstrated through static dipole polarizability calculations.

Contribution

It develops an arbitrary-order response formalism in FCIQMC, allowing systematic convergence of response properties for challenging correlated systems.

Findings

Resolved a discrepancy between coupled-cluster response results

Demonstrated stability and unbiased nature of the method

Accurately computed static dipole polarizability for molecular systems

Abstract

We formulate a general, arbitrary-order stochastic response formalism within the Full Configuration Interaction Quantum Monte Carlo framework. This modified stochastic dynamic allows for the exact response properties of correlated multireference electronic systems to be systematically converged upon for systems far out of reach of traditional exact treatments. This requires a simultaneous coupled evolution of a response state alongside the zeroth-order state, which is shown to be stable, non-transient and unbiased. We demonstrate this with application to the static dipole polarizability of molecular systems, and in doing so, resolve a discrepancy between restricted and unrestricted high-level coupled-cluster linear response results which were the high-accuracy benchmark in the literature.