PERFORMANCE OF SINGLE-REFERENCE COUPLED-CLUSTER METHODS FOR QUASI-DEGENERATE PROBLEMS - THE H-4 MODEL

Several coupled-cluster methods based on a single-determinantal reference function have been applied to the model system composed of four hydrogen atoms in a trapezoidal arrangement. For nondegenerate regions all methods with the exception of CCD provide results within 1 mhartree of the exact (FCI) value. For degenerate regions such an accuracy can be achieved with the inclusion of the T3 and T4 clusters, in an iterative and in a noniterative manner. We report results for CCSDT, CCSDTQ-1, CCSD + TQ*(CCSD), CCSD + Q(CCSDT) plus other methods. In particular, the ACP method which has been proposed to indirectly account for T4 terms is critically analyzed by including all T3 contributions.