Abstract

Stochastic electronic structure methods, such as Full Configuration Interaction Quantum Monte Carlo and Coupled Cluster Monte Carlo, have become popular in recent years as they significantly improve computational and memory costs compared to their determininstic counterparts. However, they suffer from much the same drawbacks, such as difficulties in treating systems with multireference character by coupled cluster. We present a novel multireference modification to the stochastic coupled cluster algorithm, based on a single reference formalism. By treating secondary references as excitations of the primary reference, while also allowing them to change the selection and spawning criteria in the Monte Carlo propagation, we obtain a method that preserves the appealing simplicity of the single-reference algorithm, while significantly improving performance for known strongly correlated systems, such as the stretched nitrogen molecule.4 For this system, a calculation using two references successfully captures the correlation energy (to within chemical accuracy) at the CCSDT level. The method can be trivially expanded to an arbitrary numbers of references, giving results that compare favourably to previous complete active space (CAS) based multireference coupled cluster methods. The ability to carefully control the included references, as well as their allowed excitation levels, provides us with a new tool to better understand the importance of CAS determinants to the quality of the predicted properties.