A coupled cluster approach with triple excitations

The coupled-cluster model for electron correlation is generalized to include the effects of connected triple excitation contributions. The detailed equations for triple excitation amplitudes are presented, and a simplified version implemented that retains the dominant terms. The model presented, CCSDT-1, provides the energy correct through fourth order and the wave function through second order. The CCSDT-1 model is illustrated by comparing with full CI results for HF, BH, and H2O, the latter at several geometries.