Abstract

In quantum chemistry we have many scalable methods for the accurate treatment of single-reference problems, notably including density functional theory and coupled cluster theory. However, these methods are less reliable when applied to strongly multi-reference problems. In such cases, a common approach is to perform a complete active space (CAS) procedure followed by perturbation theory. However, solving this CAS problem is challenging beyond small active spaces. In this talk, we will investigate variational Monte Carlo (VMC) and the fixed-node approximation as an approach for solving the active space problem. Specifically, we will use VMC to optimise Jastrow mean-field wave functions, which can provide an efficient representation of multi-reference states. We then investigate full configuration interaction quantum Monte Carlo (FCIQMC) as a method to improve the accuracy of these VMC solutions further, and investigate the scaling of this approach.