Feynman path integral-ab initio investigation of the excited state properties of C2H4

A tight binding based Feynman path integral Monte-Carlo approach has been c ombined with an ab initio configuration interaction scheme to study the exc ited singlet states of C2H4 under consideration of the nuclear degrees of f reedom. Transition energies and oscillator strengths, which have been avera ged over manifolds of nuclear configurations, are compared with single-poin t values calculated at the minimum of the potential energy. The quantum flu ctuations of the nuclei cause a reduction of the transition energies and a complete redistribution in the transition intensities. Transitions, which a re dipole allowed in the rigid D-2h geometry of ethylene, lose intensity un der the influence of the nuclear fluctuations; vice versa for transitions t hat are dipole forbidden under D-2h symmetry. (C) 2000 Elsevier Science B.V . All rights reserved.