VIBRONIC CONTRIBUTION TO STATIC MOLECULAR HYPERPOLARIZABILITIES

A simplified semiempirical scheme (AM1) is developed as a handy tool t o calculate vibrational corrections to hyperpolarizabilities of modera tely large planar conjugated systems. In formulating the scheme Born-O ppenheimer (BO) approximation is assumed and only those normal modes a re considered which correspond to vibration along the direction of the field. Ab initio calculations are performed to calibrate and justify the various steps involved in the scheme. Using this approach vibronic corrections to the third-order polarizabilities gamma are computed fo r a number of systems including the polyenes C2nH2n+2, n = 3, 4 and 5. The corrections are large and may explain some of the discrepancies t hat are found to exist between experiment and theory.