Calculation of the conformational dependence of valence and Rydberg states in n-tetrasilane

Previous CIS/6-31G** and CASSCF/6-31G* calculations of valence excited states of n-tetrasilane accounted qualitatively for the observed conformational dependence of condensed-phase UV absorption spectra. In an attempt to understand this result, we have performed CIS calculations with a larger basis set (MC-311G(2d) on Si, 6-311G on H, and 2s, 2p, and 2d diffuse orbitals at molecular center of mass). The first two excited states are of valence character at all dihedral angles if the molecule is isolated. When it is embedded in a rare gas cluster, the lowest four states are of valence character, and the results are nearly identical with those obtained without diffuse orbitals in the basis set, since all Rydberg states are shifted to higher energies relative to valence states. Similar behavior was observed for ethylene and disilene, and basis sets without diffuse functions seem appropriate for the computation of low-energy parts of condensed-phase spectra. Valence vs. Rydberg character was judged from the total [R-2] values, the Mulliken population of molecule-centered diffuse functions, and the [r(2)](1/2) values of the two 'singly occupied' natural orbitals). (C) 1999 Elsevier Science B.V. All rights reserved.