PERTURBATIVE TRIPLE EXCITATION CORRECTIONS TO COUPLED-CLUSTER SINGLESAND DOUBLES EXCITATION-ENERGIES

The contributions from various excitation levels to excitation energie s calculated within a coupled cluster framework are analyzed in terms of order in the fluctuation potential. In particular, the role of trip le excitations is considered, focusing on their importance for describ ing excitations of single and double replacement dominated character. Several noniterative triples corrections to the coupled cluster single s and doubles (CCSD) excitation energies are proposed. In the CCSDR(3) approach, which is a noniterative analog to the recently proposed ite rative CC3 model, single replacement dominated excitations are correct through third order in the fluctuation potential, and double replacem ent dominated excitations are correct through second order. The perfor mance of CCSDR(3) is compared to other noniterative and iterative trip les models in benchmark calculations on CH+, Ne, BH, and CH2. (C) 1996 American Institute of Physics.