A. Pasquarello et A. Quattropani, APPLICATION OF VARIATIONAL TECHNIQUES TO TIME-DEPENDENT PERTURBATION-THEORY, Physical review. B, Condensed matter, 48(8), 1993, pp. 5090-5094
Linear-response functions and second-order transition amplitudes can b
e calculated exactly provided one effective state, the solution of an
inhomogeneous Schrodinger equation, is known. We show how variational
principles can be applied to the calculation of this effective state;
an important advantage is the possibility of using variational techniq
ues, which are very efficient when large basis sets (such as plane wav
es) are used, or when the solution is required as a function of a free
parameter (for instance, the frequency in dynamical response function
s). The variational principle applies to solid-state as well as to ato
mic problems. We illustrate the use of one variational technique in th
e simple case of two-photon transitions in the hydrogen atom.