Partially linearized external models to active-space coupled-cluster through connected hextuple excitations

Partially linearized external models to active-space coupled-cluster through hextuple excitations, for example, CC{SDtqph}(L), CCSD{tqph}(L), and CCSD{tqph}(hyb,) are implemented and compared with the full active-space CCSDtqph. The computational scaling of CCSDtqph coincides with that for the standard coupled-cluster singles and doubles (CCSD), yet with a much large prefactor. The approximate schemes to linearize the external excitations higher than doubles are significantly cheaper than the full CCSDtqph model. These models are applied to investigate the bond dissociation energies of diatomic molecules (HF, F-2, CuH, and CuF), and the potential energy surfaces of the bond dissociation processes of HF, CuH, H2O, and C2H4. Among the approximate models, CCSD{tqph}(hyb) provides very accurate descriptions compared with CCSDtqph for all of the tested systems. (c) 2018 Wiley Periodicals, Inc.