Size-extensive effective Hamiltonian formalisms using quasi-Hilbert and quasi-Fock space strategies with incomplete model spaces

Abstract

An open-shell coupled cluster (CC) theory is developed using an incomplete model space (IMS) that works entirely within one particular n-valence Hilbert space sector. It is proved that the theory is size-extensive since both H_eff and the energies obtained therefrom are connected. The method may be viewed as the Fock space CC theory for an IMS projected onto a given Hilbert space, and is denoted here as a "quasi-Hilbert" space theory. A "quasi-Fock" formalism is also outlined where additional information on the lower valence model spaces are deliberately retained.