CALCULATION, WITH THE INCLUSION OF VIBRATIONAL CORRECTIONS, OF THE DC-ELECTRIC-FIELD-INDUCED 2ND-HARMONIC-GENERATION HYPERPOLARIZABILITY OFMETHANE

Calculations of <(gamma)over bar>(-2 omega;omega,omega,0) for CH4 are reported for a number of frequencies(omega). This mean frequency-depen dent second hyperpolarizability governs the nonlinear optical process: dc-electric-field-induced second-harmonic generation. Two levels of c alculation have been carried out: one is based on the random phase app roximation and the other on the multiconfigurational random phase appr oximation using a complete active space self-consistent-field wavefunc tion. This is the first calculation of <(gamma)over bar>(-2 omega;omeg a,omega,0) that has been made for CH4 using electron-correlated wavefu nctions. Included in the calculations are corrections for vibrational effects of which the zero-point vibrational-averaging correction is bo th dominant and significant (13-18% of the electronic values). Compari son with experiment shows that not all the electron correlation is cap tured by our treatment. The dispersion properties are also discussed. (C) 1997 American Institute of Physics. [S0021-9606(97)02244-7].