Active space approaches combining coupled-cluster and perturbation theory for ground states and excited states

Abstract

We evaluate the performance of approaches that combine coupled-cluster and perturbation theory based on a predefined active space of orbitals. Coupled-cluster theory is used to treat excitations that are internal to the active space and perturbation theory is used for all other excitations, which are at least partially external to the active space. We consider a variety of schemes that differ in how the internal and external excitations are coupled. Such approaches are presented for ground states and excited states within the equation-of-motion formalism. Results are given for the ionisation potentials and electron affinities of a test set of small molecules and for the correlation energy and band gap of a few periodic solids.