The evaluation of two-photon absorptivities in molecules in the adiabatic approximation by CI expansion

We stress the advantages of solving directly the equations of perturbation theory by CI expansion instead of using the sum-over states formulation of two-photon absorption. A formula to compute the matrix element entering the expression for the cross section taking into account the nuclear degrees of freedom is derived from the Karplus-Kolker variational principle. Numerical applications to H2 are presented.