Pg. Szalay, ANALYTIC ENERGY DERIVATIVES FOR COUPLED-CLUSTER METHODS DESCRIBING EXCITED-STATES - GENERAL FORMULAS AND COMPARISON OF COMPUTATIONAL COSTS, International journal of quantum chemistry, 55(2), 1995, pp. 151-163
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.