RESPONSE THEORIES BASED ON A STATE-SPECIFIC MULTI-REFERENCE COUPLED CLUSTER FORMALISM

We present in this paper linear response theories based on our recently formulated state-specific multi-reference coupled cluster (SS-MRCC) formalism with a complete active space (CAS). The coupled cluster Ansatz for the wave operator excites each of the reference determinants to all the virtual functions via its own cluster operator. The linear dependence of the cluster operators implicit in such a choice is eliminated by positing suitable sufficiency conditions while ensuring size-extensivity. The use of a CAS then also guarantees size-consistency. After a resume of the SS-MRCC method, which serves both to highlight the essentials and to introduce the notations, we discuss two distinct response formulations. The first is a straightforward numerically oriented approach exploiting our SS-MRCC to compute static second order properties via perturbed orbitals in a finite field. The other, a genuinely analytical linear response theory using excitation operators, generalizes the single reference CC-based linear response theory to a multi-reference function for obtaining excitation energies. Both these formulations are illustrated with example applications.