O. Christiansen et al., RESPONSE FUNCTIONS FROM FOURIER COMPONENT VARIATIONAL PERTURBATION-THEORY APPLIED TO A TIME-AVERAGED QUASI-ENERGY, International journal of quantum chemistry, 68(1), 1998, pp. 1-52
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.