Higher Energy Derivatives in Hilbert Space Multi- Reference Coupled Cluster Theory : A Constrained Variational Approach

In this paper, we present formulation based on constrained variational approach to compute higher energy derivatives upto third order in Hilbert Space MultiReference Coupled Cluster (HSMRCC) Theory. This is done through the use of a functional with Lagrange multipliers corresponding to HSMRCC method, as done by Helgaker, Jorgensen and Szalay. We derive explicit expressions upto third order energy derivatives. Using (2n + 1) and (2n + 2) rules, the cancellation of higher order derivatives of functional parameters that are not necessary according to these rules, is explicitly demonstated. Simplified expressions are presented. We discuss several aspects of the functional used and its potential implications.