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Int. J. Mol. Sci. 2002, 3(6), 710-732; doi:10.3390/i3060710
Article

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

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Received: 18 December 2001 / Accepted: 10 April 2002 / Published: 30 June 2002
(This article belongs to the Special Issue Multireference Quantum Chemical Methods)
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Abstract

In this paper, we present formulation based on constrained variational approach to compute higher energy derivatives upto third order in Hilbert Space Multi-Reference 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.
Keywords: Energy Derivatives; Lagrange Multipliers; Constrained Variation; Hilbert Space Multi-Reference; Coupled Cluster Energy Derivatives; Lagrange Multipliers; Constrained Variation; Hilbert Space Multi-Reference; Coupled Cluster
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Shamasundar, K.R.; Pal, S. Higher Energy Derivatives in Hilbert Space Multi-Reference Coupled Cluster Theory : A Constrained Variational Approach. Int. J. Mol. Sci. 2002, 3, 710-732.

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