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

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 'twinning' ionized states, the 2(2)A(g) and 3(2)A(g) states, and the peak at 27.4 eV is attributed to the 6(2)A(g) and 7(2)A(g), states. In the energy region around 31 eV. some ionized states are suggested; to locate with small intensities. The 1(2)B(2g) State obtains its intensity by the initial state configuration interaction. (C) 1998 Elsevier Science B.V.