ANALYSIS OF THE VIBRATIONAL, STATIC AND DYNAMIC, 2ND HYPERPOLARIZABILITY OF 5 SMALL MOLECULES

Various approximate methods for the calculation of vibrational second hyperpolarizabilities are assessed on the basis of results or CH4, NH3 , H2O, HF, and CO2. Three specific variations of the perturbation-theo retic approach are analyzed. These are defined by the types of terms r etained in expansions which involve electrical and mechanical anharmon icities. The lowest-order approximation being the double-harmonic one and the next higher one the relaxation method. For the dynamic hyperpo larizabilities, the replacement of the optical frequencies (omega) by omega-->(infinity) (the infinite-frequency approximation) is also expl ored. On the evidence at hand it is concluded that the relaxation/infi nite-frequency method is a satisfactory approximation for the nonlinea r optical vibrational hyperpolarizabilities. (C) 1996 American Institu te of Physics.