Theoretical study on the ionized states of ethylene by the SAC-CI (general-R) method Original Research Article

The SAC (symmetry adapted cluster)/SAC-CI (configuration interaction) general-R method is successfully applied to the photoelectron spectrum of ethylene. The theoretical spectrum satisfactorily reproduces the outer- and inner-valence regions of the spectrum. The exponential generation (EG) algorithm followed by perturbation selection (PS) is shown to be useful in the generation of small and yet effective higher-excitation operators for the SAC-CI general-R method. The peak at 23.7 eV is assigned to the `twinningʹ ionized states, the 2 2Ag and 3 2Ag states, and the peak at 27.4 eV is attributed to the 6 2Ag and 7 2Ag states. In the energy region around 31 eV, some ionized states are suggested to locate with small intensities. The 1 2B2g state obtains its intensity by the initial state configuration interaction.