Investigation of an asymmetric triple-excitation correction for coupled-cluster energies

摘要

A correction for the effects of connected triple excitations to the coupled-cluster singles and doubles energy is studied. The approach relies on the fact that the ground-state coupled-cluster energy may be viewed as an eigenvalue of an effective (similarity transformed) Hamiltonian with associated left and right eigenvectors. Taking these as zeroth-order wave functions and using a conventional partitioning of the bare electronic Hamiltonian, the lowest order triple-excitation correction to the correlation energy is found to have an asymmetric form that involves cluster amplitudes as well as components of the left eigenvector. The popular (T) correction may be viewed as an approximation to the present approach, though the latter is approximately a factor of 2 more expensive to compute. The method is applied to a number of difficult cases, including the harmonic vibrational frequencies of ozone and the equilibrium bond length of N-2. In addition, the theory of analytic gradients for the method is outlined and some aspects regarding its implementation are discussed. (C) 1998 John Wiley & Sons, Inc. [References: 65]