FREQUENCY-DEPENDENT HYPERPOLARIZABILITIES WITH APPLICATION TO FORMALDEHYDE AND METHYL-FLUORIDE

Methods for the determination of the frequency dependent hyperpolarizabilities, beta( -2-omega;omega,omega), beta( -omega;omega,0), and beta(0;omega, - omega) at the self-consistent field (SCF) level of theory are discussed and compared. Two efficient alternatives are proposed; one involves determination of appropriate response vectors which arise in first-order perturbation theory at frequency omega, and the other involves solving first-order perturbation theory equations at frequency 0 (static), omega and 2-omega. Neither approach involves solution of the second-order perturbation theory equations. The method is illustrated with application to formaldehyde and methyl fluoride using large one-particle basis sets. The basis set requirement for convergence of this property at the SCF level of theory is investigated. Basis sets including up to d functions on first row atoms and rho functions on hydrogen incorporating diffuse functions appear to be adequate. The effect of electron correlation on the static hyperpolarizability is estimated using second-order Moller-Plesset theory. Comparison is made with experimental values where available.