MULTIPHOTON TRANSITION MOMENTS AND ABSORPTION CROSS-SECTIONS IN COUPLED-CLUSTER RESPONSE THEORY EMPLOYING VARIATIONAL TRANSITION-MOMENT FUNCTIONALS

Based on an analysis of the first residues of coupled cluster response functions we devise variational functionals from which the transition moments for n-photon excitations can be calculated as nth derivatives . Combining these functionals with variational perturbation theory, we obtain a new approach for the derivation of multiphoton transition mo ments which allows us to utilize the full strengths of variational per turbation theory without the roundabout way via residues of response f unctions. Coupled cluster multiphoton transition moments derived by th is approach are formally equivalent to the one identified from the fir st residues of the ground state response functions. The introduction o f the variational functionals makes the mathematical structure of the transition moments more transparent and provides an interpretation of intermediates in terms of responses of excited state vectors and Lagra ngian multipliers. 2n + 1 and 2n + 2 rules are formulated for the tran sition moments and build the basis for a straightforward derivation of a computational efficient formulation. The strength of the new approa ch is demonstrated by the derivation of three- and four-photon transit ion moments. (C) 1998 American Institute of Physics. [S0021-9606(98)02 520-3]