Efficient calculation of Franck-Condon factors and vibronic couplings in polyatomics

We present a technique for the calculation of Franck-Condon factors and other integrals between vibronic wave functions belonging to different electronic states. The technique is well suited for the determination of the nonadiabatic or spin-orbit couplings related to radiationless decays in polyatomics. Rigorous or approximate partitions of the internal coordinate space are exploited to achieve better efficiency and/or to go beyond the harmonic approximation. The technique is tested by computing the Internal Conversion and InterSystem Crossing rates of (CH3)(3)CNO in its (1)(n --> pi*) state. (C) 2001 John Wiley & Sons, Inc.