ANALYTIC ENERGY DERIVATIVES FOR COUPLED-CLUSTER METHODS DESCRIBING EXCITED-STATES - GENERAL FORMULAS AND COMPARISON OF COMPUTATIONAL COSTS

It is possible to derive energy derivatives for nonvariational (e.g., coupled-cluster) methods invoking the generalized Hellmann-Feynman the orem. In such a procedure, one constructs a functional which, besides the usual wave-function parameters, contains new ones. One set of stat ionary conditions will reproduce exactly the original equations of the method, while the others will determine the value of the new paramete rs. We applied this straightforward procedure to derive analytic energ y derivatives for several coupled-cluster (CC) methods applicable to e xcited states such as the Hilbert-space CC method, two-determinetal (T D) CC method, Fock-space CC method, and equation-of-motion-CC (EOM-CC) method. Finally, we compared the computational requirements for the d ifferent methods. (C) 1995 John Wiley and Sons, Inc.