FREQUENCY-DEPENDENT HYPERPOLARIZABILITIES OF POLYYNES

Ab initio calculations have been performed for the static and dynamic polarizability and hyperpolarizability for a series of polyynes C2nH2 using self-consistent field (SCF) (n = 1-6) and multiconfiguration sel f-consistent field (MCSCF) (n = 1-4) wave functions. We have considere d the longitudinal component of the static hyperpolarizability and the same component of the dynamic hyperpolarizability measured in electri c-field induced second harmonic generation gamma(ESHG) = gamma(-2omega ;omega,omega,0). The frequency dependence of the polarizability and hy perpolarizability has been rationalized in terms of the coefficients i n expansions in omega2. The static hyperpolarizabilities vary smoothly with the chain length and satisfy gamma (C2nH2) = gamma(C2H2) X n(X), where X almost-equal-to 3.0. The dynamic hyperpolarizability satisfie s a similar relation where X in creases slowly with omega.