Previous CIS/6-31G** and CASSCF/6-31G* calculations of valence excited stat
es of n-tetrasilane accounted qualitatively for the observed conformational
dependence of condensed-phase UV absorption spectra. In an attempt to unde
rstand 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 cha
racter at all dihedral angles if the molecule is isolated. When it is embed
ded in a rare gas cluster, the lowest four states are of valence character,
and the results are nearly identical with those obtained without diffuse o
rbitals in the basis set, since all Rydberg states are shifted to higher en
ergies relative to valence states. Similar behavior was observed for ethyle
ne 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) val
ues of the two 'singly occupied' natural orbitals). (C) 1999 Elsevier Scien
ce B.V. All rights reserved.