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Abstract

It is possible to derive energy derivatives for nonvariational (e.g., coupled-cluster) methods invoking the generalized Hellmann–Feynman theorem. In such a procedure, one constructs a functional which, besides the usual wave-function parameters, contains new ones. One set of stationary conditions will reproduce exactly the original equations of the method, while the others will determine the value of the new parameters. We applied this straightforward procedure to derive analytic energy derivatives for several coupled-cluster (CC) methods applicable to excited states such as the Hilbert-space CC method, two-determinetal (TD) CC method, Fock-space CC method, and equation-of-motion–CC (EOMCC) method. Finally, we compared the computational requirements for the different methods. © 1995 John Wiley & Sons, Inc.