Theory and computational methods for studies of nonlinear phenomena in laser spectroscopy. II. Calculations of steady‐state wave functions

Methods will be developed for variational calculations of steady‐state wave functions and the corresponding quasienergies. These functions appear as solutions to the time‐dependent Schrödinger equation for a molecule interacting with an oscillating electromagnetic field. A variation principle is used to obtain coupled equations for each Fourier component ( FC ) of the steady‐state wave function. The FC s are expanded in configuration state functions using the structure of the complete active space ( CAS ) SCF approach. Two methods are outlined, one which is based on a common set of molecular orbitals for all FC s and one based on an individual optimization of both CI coefficients and molecular orbitals for each FC . The latter method leads to molecular orbital sets for the different FC s which are mutually nonorthogonal. Formulas are derived for the calculation of higher‐order dynamic susceptibilities for steady states. An application is made to the H 2 molecule, and results are presented for the frequency‐dependent polarizability and hyperpolarizability along the molecular axis. The static values obtained are α = 6.413 (6.387) and γ = 676 (674) atomic units, with exact numbers within parentheses.