A size-extensive state-specific multi-reference many-body approach using incomplete model spaces

We present a size-extensive and size-consistent state-specific multi-reference coupled-cluster approach based on an incomplete model space (IMS) and also discuss simplifications if the IMS is quasi-complete. Methods such as perturbation theory or CEPA-like schemes follow naturally as suitable approximants. Intermediate normalization for the wave-operator, Ω is abandoned in the formalism and suitable excitations, defined as open and quasi-open – which excite out of the IMS by their action on at least one of the model functions – are incorporated in Ω. The effective operator W, leading to the energy on diagonalization, is a closed connected operator.