A CASPT2 STUDY OF THE VALENCE AND LOWEST RYDBERG ELECTRONIC STATES OFBENZENE AND PHENOL

The valence excited states and the 3s, 3p, and 3d (united atom) Rydber g states of benzene and phenol have been obtained by the CASPT2 method , which computes a second-order perturbation correction to complete ac tive space self-consistent field (CASSCF) energies. All non-zero dipol e oscillator strengths are also computed, at the CASSCF level. For ben zene, 16 singlet and 16 triplet states with excitation energies up to ca. 7.86 eV (63 400 cm(-1)) are obtained. Of these, 12 singlet anti th ree triplet energies are experimentally known well enough to allow mea ningful comparison. The average error is around 0.1 eV. The highest of these singlet states (2(1)E(2g)) is the highest valence pi pi state predicted by elementary x-electron theory. Its energy is then consider ably lower than has been suggested from laser flash experiments, but i n perfect agreement with a reinterpretation of that experiment. For ph enol, 27 singlet states are obtained, in the range 4.53-7.84 eV (63 30 0 cm(-1)). Only the lowest has a well-known experimental energy, which agrees with the computed result within 0.03 eV. The ionization energy is in error by 0.05 eV.