RESPONSE FUNCTIONS FROM FOURIER COMPONENT VARIATIONAL PERTURBATION-THEORY APPLIED TO A TIME-AVERAGED QUASI-ENERGY

It is demonstrated that frequency-dependent response functions can con veniently be derived from the time-averaged quasienergy. The variation al criteria for the quasienergy determines the time-evolution of the w ave-function parameters and the time-averaged time-dependent Hellmann- Feynman theorem allows an identification of response functions as deri vatives of the quasienergy. The quasienergy therefore plays the same r ole as the usual energy in time-independent theory, and the same techn iques can be used to obtain computationally tractable expressions for response properties, as for energy derivatives in time-independent the ory. This includes the use of the variational Lagrangian technique for obtaining expressions for molecular properties in accord with the 2n + 1 and 2n + 2 rules. The derivation of frequency-dependent response p roperties becomes a simple extension of variational perturbation theor y to a Fourier component variational perturbation theory. The generali ty and simplicity of this approach are illustrated by derivation of li near and higher-order response functions for both exact and approximat e wave functions and for both variational and nonvariational wave func tions. Examples of approximate models discussed in this article are co upled-cluster, self-consistent field, and second-order Moller-Plesset perturbation theory. A discussion of symmetry properties of the respon se functions and their relation to molecular properties is also given, with special attention to the calculation of transition-and excited-s tate properties. (C) 1998 John Wiley & Sons, Inc.