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

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 χi from the reference function /gf, and the latter excites to χi via intermediates /gfμ 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.