Molecular applications of size-extensive quasi-Hilbert- and quasi-Fock-space coupled-cluster formalisms using incomplete model spaces

Abstract

In this paper, we report the first numerical applications of the size-extensive quasi-Hilbert- and quasi-Fock-space coupled-cluster theories, developed by us recently, to the ground and the first excited ^1,3 Σ states of the molecule LiH. Unlike the corresponding Fock-space theories, there is no need to solve for the cluster amplitudes of lower-valence model spaces characterizing LiH⁺ and LiH^- species in these formulations. The results are compared with the CISD values, those from the Fock-space theories and the coupled-cluster-based linear-response theory, and are found to be quite satisfactory. This indicates the computational viability of the two formulations.