An augmented coupled cluster method and its application to the first-row homonuclear diatomics

An augmented coupled cluster scheme to evaluate the higher order electron correlation effects is proposed. The method is carried out in two steps. First, a coupled cluster calculation with all double substitutions (CCD) is performed. The converged CCD wave function is then used in the evaluation of the contribution of single and triple substitutions. The method is correct to fourth order in a perturbation expansion and includes significant fifth and higher order terms. Illustrative calculations on the excitation and dissociation energies of first-row homonuclear diatomic molecules are reported. The low-lying excitation energies of B2 and C2 are accurately calculated. The dissociation energies of B2, C2, N2, O2, and F2 are all uniformly underestimated by 0.1–0.3 eV using large spdf basis sets.