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 combined with an ab initio configuration interaction scheme to study the excited singlet states of C2H4 under consideration of the nuclear degrees of freedom. Transition energies and oscillator strengths, which have been averaged over manifolds of nuclear configurations, are compared with single-point values calculated at the minimum of the potential energy. The quantum fluctuations of the nuclei cause a reduction of the transition energies and a complete redistribution in the transition intensities. Transitions, which are dipole allowed in the rigid D2h geometry of ethylene, lose intensity under the influence of the nuclear fluctuations; vice versa for transitions that are dipole forbidden under D2h symmetry.