A Power Series Approximation in Symmetry Projected Coupled Cluster Theory

TL;DR

This paper introduces a novel power series approximation in symmetry projected coupled cluster theory, aiming to unify the treatment of strong and weak electron correlations in quantum chemistry.

Contribution

It presents an alternative formulation to identify disentangled cluster operators, facilitating the combination of symmetry projection with coupled cluster methods.

Findings

Promising results on model systems

Effective handling of strong correlations

Potential for improved weak correlation description

Abstract

Projected Hartree-Fock theory provides an accurate description of many kinds of strong correlations but does not properly describe weakly-correlated systems. On the other hand, single-reference methods such as configuration interaction or coupled cluster theory can handle weakly-correlated problems but cannot properly account for strong correlations. Ideally, we would like to combine these approaches in a symmetry-projected coupled cluster approach, but this is far from straightforward. In this work, we provide an alternative formulation to identify the so-called disentangled cluster operators which arise when we combine these two methodological strands. Our formulation shows promising results for model systems and small molecules.