Excited states and ionized states of aromatic ring compounds and model studies of chemisorption of H/sub 2/ on platinum studied by SAC/SAC-CI method

This talk consists of two topics. One is on the valence and Rydberg excitations and ionizations of aromatic ring compounds (benzene, pyridine, furan, pyrrole, etc.) studied by SAC and SAC-CI method within large active spaces. For the valence π-π excitations, particularly for the so-called V states, the σ-reorganization effect is very important. The 35π+45σ SAC-CI calculations improves the excitation energies for the T/sub 3/(/sup 3/B/sub 2μ/), S/sub 2/(/sup 1/B/sub 1μ/), and S/sub 3/(/sup 1/E/sub 1μ/) states of benzene by about 1 eV, in comparison with the 23π MR-CI (SDT-CI) result of Hay and Shavitt. The discrepancy from the experimental value still existing is 0.42, 0.40, and 0.52 eV, respectively. The Rydberg excited states are studied systematically for both π and σ states. For benzene, the SAC-CI method reproduces the experimental values to within 0.3 eV. the second topic is on the cluster model studies of chemisorption of H/sub 2/ on a platinum surface. Various orientations and interaction modes of H/sub 2/ on Pt/sub n/ are studied. The H/sub 2/ molecule is dissociatively adsorbed through a side-on, on-top orientation on a Pt atom of a cluster. Single pt atom is essential for dissociative adsorption, but afterwards, the migration of H on a surface occurs almost without barrier. The adsorption energy is improved when the cluster size is enlarged