The construction of a size-extensive intermediate Hamiltonian in a coupled-cluster framework

Abstract

A size-extensive formulation for an intermediate Hamiltonian H int, furnishing size-extensive energies for the main roots, is presented. The working model space, comprised of the main and intermediate space, is taken as complete. The starting point is a shifted Bloch equation derived by us, involving shift operators generated by the projector spanned by the intermediate eigenvectors of H int. It is shown that a manifestly size-extensive H int can be constructed in the coupled-cluster framework provided we choose the shift operator explicitly as additively separable and the associated intermediate wave operator R as multiplicatively separable.