An assessment for the full coupled cluster method including all single, double, and triple excitations: The diatomic molecules LiH, Li2, BH, LiF, C2, BeO, CN+, BF, NO+, and F2

Abstract

The full coupled cluster singles, doubles, and triples (CCSDT) model for molecular electronic structure theory has been used in conjunction with two less complete coupled cluster methods (CCSD, CCSDT-1) to predict the equilibrium bond distances and harmonic vibrational frequencies of a family of closed-shell ground state diatomic molecules. The results are compared with the configuration interaction method including all single, double, triple, and quadruple excitations (CISDTQ) and experiment. With previous CCSDT results for HF, OH−, N2, and CO, the present work provides a realistic assessment of the CCSDT method. The BeO molecule in particular places great demands on the theory, and the linearized triple excitation model CCSDT-1 fails badly.