Treatment of quasi-degeneracy in single-reference coupled-cluster theory. Separation of dynamical and nondynamical correlation effects

Abstract

We show that the most important nondynamical correlation effects of quasidegeneracy arising, for example, out of bond-breaking, can be effectively handled using low-body cluster operators (usually up to rank two) via a modification of single-reference coupled-cluster (CC) theory. This is done by separating the effects of cluster operators leading to dynamical and nondynamical correlations. The former leads to direct excitations to the virtual functions χ_i from the reference function /gf, and the latter excites to χ_i via intermediates /g^fμ quasi-degenerate with /gf. By utilizing a separability condition for the dynamical and nondynamical correlation energies, a rigorously size-extensive and size-consistent CC theory is developed which should be well-behaved and accurate at bond-breaking geometries.