The eigenvalue-independent partitioning technique in Fock space: an alternative route to open-shell coupled-cluster theory for incomplete model spaces

Abstract

Our recent open-shell coupled-cluster (CC) theory for incomplete model spaces, having valence holes and valence particles, is cast in an alternative form having computational advantages. An eigenvalue-independent partitioning technique in Fock space converts the CC equations for each m-hole, n-particle model space of dimension N^(m,n) to a non-Hermitian eigenproblem of larger dimension, whose N^(m,n) roots are the energies of interest. This alternative strategy works even in the presence of intruders, as is demonstrated by applying it to the main and satellite peaks of the IP and Auger spectra of HF and to the excitation energies of N₂.