The two lowest bands, B-1(2u) and B-1(1u), of the electronic spectrum of th
e benzene molecule have been studied theoretically using a new method to co
mpute vibronic excitation energies and intensities. The complete active spa
ce (CAS) self-contained field (SCF) method (with six active pi-orbitals) wa
s used to compute harmonic force field for the ground state and the B-1(2u)
and B-1(1u) electronic states. A linear approximation has been used for th
e transition dipole as a function of the nuclear displacement coordinates.
Derivatives of the transition dipole were computed using a variant of the C
ASSCF state interaction method. Multiconfigurational second-order perturbat
ion theory (CASPT2) was used to obtain absolute excitation energies (12 act
ive pi-orbitals). The results show that the approach works well. Vibrationa
l progressions are well described in both bands and intensities, and energi
es are in agreement with experiment, in particular when CASPT2 derived geom
etries are used. One interesting result is that computed vertical energies
fall about 0.1 eV on the high energy side of the band maximum. (C) 2000 Ame
rican Institute of Physics. [S0021-9606(00)30306-3].