Externally Corrected Coupled-Cluster Approaches: Energy versus Amplitude Corrected CCSD

The externally corrected coupled-cluster methods with singles and doubles (ecCCSD), which exploit some independently available wave function as a source of higher-than-pair clusters, are considered. The focus is on methods that employ a modest-size multireference (MR) configuration interaction (with singles and doubles, CISD) wave function as the external source. Both the amplitude- and energy-corrected CCSD methods are employed, the former correcting the standard single reference (SR) CCSD equations for triples and quadruples, while the latter accounts for the nondynamic correlation effects when evaluating the energy by employing the MR CISD wave function in lieu of the single determinantal (usually Hartree-Fock) reference in the asymmetric energy formula. The performance and relationship of both types of approaches is illustrated by computing the rotational and vibrational energy levels using the potential generated by these various methods and by comparing the calculated spectra with the experimental ones for the simplest first-row hydride, namely the LiH molecule. A special attention is paid to the role of core-correlation effects, in which case we also consider the HF molecule.