Ground state and excited state properties of ethylene isomers studied by acombined Feynman path integral - ab initio approach

Ground state and excited state properties of ethylene, C2H4, and several et hylene isomers have been studied by Feynman path integral Monte Carlo (PIMC ) simulations. The PIMC treatment of the atomic nuclei has been combined wi th different electronic Hamiltonians in order to analyse the influence of t he nuclear degrees of freedom on electronic quantities. Electronic expectat ion values at the minimum of the potential energy surface (PES) have been c ompared with PIMC based ensemble averaged values. Ensemble averaged quantit ies have been derived by Hamiltonians of the ab initio type and a tight-bin ding (TB) one-electron model. The combined influence of anharmonicities in the interatomic potential and the quantum fluctuations of the atomic nuclei lead to ensemble averaged bondlengths r(g) which are significantly larger than the parameters r(e) at the minimum of the PES. The implications of thi s bond length elongation for the electronic properties of ethylene are disc ussed. The occupied canonical molecular orbitals (CMOs) of ethylene are des tabilized under the influence of the nuclear degrees of freedom while virtu al CMOs are stabilized. These shifts of one-electron energies suggest a com parison of electronic excitation energies at the minimum of the PES with PI MC based ensemble averages. The quantum fluctuations of the nuclei cause a strong redistribution in the intensities of electronic transitions. Transit ions, which are dipole allowed in the planar D-2h geometry of ethylene, los e intensity under the influence of nuclear quantum effects, and vice versa for electronic excitations that are dipole forbidden under D-2h symmetry. T his 'vibrational borrowing' is enhanced with decreasing atomic masses. The Feynman centroid density has been used to calculate the anharmonic vibratio nal wavenumbers of C2H4 and C2D4. The results of the present PIMC simulatio ns have been employed to emphasize general problems of electronic structure calculations based on a single nuclear configuration (i.e. the configurati on at the minimum of the PES).