A coupled cluster approach with triple excitations

Abstract
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.