Cluster expansion of the wave function. Valence and Rydberg excitations and ionizations of ethylene

The symmetry-adapted-cluster (SAC) and SAC-CI theories have been applied successfully to the calculations of the valence and Rydberg excitations and the outer- and inner-valence ionizations of ethylene. Among all, the result for the singlet (π,π*) excited state is very encouraging, showing the effectiveness of the SAC-CI theory for the descriptions of electron correlations in the excited state. A systematic assignment of the singlet and triplet Rydberg excited states is given. The outer-valence ionization spectra are well reproduced by the SAC-CI theory, though in the inner-valence region, the intensities of the calculated ionization peaks are rather weak in comparison with the experiment.