INVESTIGATION OF AN ASYMMETRIC TRIPLE-EXCITATION CORRECTION FOR COUPLED-CLUSTER ENERGIES

A correction for the effects of connected triple excitations to the co upled-cluster singles and doubles energy is studied. The approach reli es on the fact that the ground-state coupled-cluster energy may be vie wed as an eigenvalue of an effective (similarity transformed) Hamilton ian with associated left and right eigenvectors. Taking these as zerot h-order wave functions and using a conventional partitioning of the ba re electronic Hamiltonian, the lowest order triple-excitation correcti on to the correlation energy is found to have an asymmetric form that involves cluster amplitudes as well as components of the left eigenvec tor. The popular (T) correction may be viewed as an approximation to t he present approach, though the latter is approximately a factor of 2 more expensive to compute. The method is applied to a number of diffic ult cases, including the harmonic vibrational frequencies of ozone and the equilibrium bond length of N-2. In addition, the theory of analyt ic gradients for the method is outlined and some aspects regarding its implementation are discussed. (C) 1998 John Wiley & Sons, Inc.