Diagonal perturbative triple corrections to the general-model-space state-universal coupled-cluster method: are they warranted and useful?

The recently developed general-model-space (GMS) state-universal (SU) coupled-cluster (CC) approach, together with its version corrected for triples via a single-reference (SR) CCSD(T)-type correction of the diagonal elements of the effective Hamiltonian, is applied to several molecular electronic structure problems in order to assess their performance and the role of triples. These results are compared with an alternative handling of higher-than-pair clusters via the externally corrected SU CCSD method, denoted (M, N)-CCSD, which employs N wave functions of the M-reference CISD as an external source for N-reference SU CCSD. These methods are applied to the problem of bond breaking in the ground and excited states of the F-2 and HF molecules, where the high-spin triplet component is also handled via the SR CCSD method. We further examine the vertical excitation energies of water and the basic spectroscopic constants (equilibrium geometries, harmonic frequencies, and excitation energies) for several low-lying states of oxygen. The results are encouraging and are discussed from the viewpoint of the applicability and usefulness of perturbative-type triple corrections.