Comparison of various Franck-Condon and vibronic coupling approaches for simulating electronic spectra: the case of the lowest photoelectron band of ethylene.

Various time independent approaches for simulating electronic absorption spectra are discussed and applied to the lowest band (10.2-11.4 eV) of the photoelectron spectrum of ethylene. The electronic structure calculations for the ionized states of ethylene are performed using the Equation of Motion Coupled Cluster method for Ionization Potentials (IP-EOM-CCSD) in a TZ2P basis set. Various Franck-Condon (FC) and vibronic coupling model Hamiltonian approaches are used to simulate the spectrum and a comparison is made to the experimental data. The potential energy surface is highly anharmonic and it is necessary to use more sophisticated FC approaches than the traditional harmonic approach to obtain a qualitatively correct simulation of the spectrum. Duschinsky rotation is found to play an important role in determining the detailed intensity pattern of the spectrum, while non-adiabatic effects are found not to be important. The spectrum is found to be very sensitive to the precise values of some of the parameters used in the vibronic model, that determine the details of the adiabatic potential energy surface.