Multireference configuration interaction and perturbation theory without reduced density matrices.

The computationally expensive evaluation and storage of high-rank reduced density matrices (RDMs) has been the bottleneck in the calculation of dynamic correlation for multireference wavefunctions in large active spaces. We present a stochastic formulation of multireference configuration interaction (MRCI) and perturbation theory (MRPT) that avoids the need for these expensive RDMs. The algorithm presented here is flexible enough to incorporate a wide variety of active space reference wavefunctions including selected configuration interaction, matrix product states, and symmetry-projected Jastrow mean field wavefunctions. It enjoys the usual attractive features of Monte Carlo methods such as embarrassing parallelizability and low memory costs. We find that the stochastic algorithm is already competitive with the deterministic algorithm for small active spaces containing as few as 14 orbitals. We illustrate the utility of our stochastic formulation using benchmark applications.