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Abstract
In this work an internally contracted Stochastic Complete Active Space Second Order Perturbation Theory, Stochastic–CASPT2, is reported. The method relies on stochastically sampled reduced density matrices (RDMs) up to rank four and con- tractions thereof with the generalized Fock matrix. A new protocol for calculating higher–order RDMs in FCIQMC has been designed based on: (1) restricting sampling of the corresponding excitations to a deterministic subspace, (2) averaging the RDMs from independent dynamics and (3) projecting the resulting RDMs onto the closest positive semi-definite matrix. Our protocol avoids previously encountered numerical conditioning problems stemming from numerical noise in the orthogonalisation of the perturber overlap matrix. The Chromium dimer CASSCF(12,12)/CASPT2 binding curve is computed as a proof of concept.
