Analytical Evaluation of Gradients in Coupled-Cluster and Many-Body Perturbation Theory

摘要

A diagrammatic derivation of the coupled cluster (CC) linear response equations for gradients is presented. Many Body Perturbation Theory (MBPT) approximations emerge as low order iterations of the CC equations. In CC theory a knowledge of the change in cluster amplitudes with displacement is required, which would not be necessary if the coefficients were variationally optimum, as in the Configuration Interaction (CI) approach. However, it is shown that the CC linear response equations can be put in a form where there is no more difficulty in evaluating CC gradients than in the variational CI procedure. This offers a powerful approach for identifying critical points on energy surfaces and in evaluating other properties.