The full CCSDT model for molecular electronic structure

The full coupled-cluster model (CCSDT) single, double, and triple excitation method defined by the wave function exp(T1+T2+T3)‖Φ0〉 is formulated and computationally implemented for the first time. Explicit computational equations are presented. The method is applied to numerous examples including BH, FH, C2H2, CO, Ne, F−, and H2O to assess its applicability to the correlation problem. Results from CCSDT agree with full CI, to an average error of less than 1 kcal/mol even for difficult bond breaking examples. A series of results for various approximate, but computationally more efficient versions of the full CCSDT model are also studied and shown to give results in excellent agreement with CCSDT. Additional comparisons with fifth-order MBPT are reported.