TOWARDS AN ACCURATE MOLECULAR-ORBITAL THEORY FOR EXCITED-STATES - THE BENZENE MOLECULE

A computational scheme is proposed for ab initio calculations of electronic spectra of molecular systems. The scheme is firmly based on the different effects that determine the excitation energies and properties of the excited states. li aims to be accurate to better than 0.5 eV for excitation energies and should provide structural and physical data for the excited states. Applications are possible to rather large molecules (up to 20 atoms) with high-quality basis sets. Extensions of the approach will be possible when direct methods have been implemented. The scheme is based on the CASSCF method, which gives a proper description of the major features in the electronic structure of the excited state, independent of its complexity, and accounts for all near-degeneracy effects and includes full orbital relaxation. Remaining dynamic electron correlation effects are added using second-order perturbation theory with the CASSCF wavefunction as the reference state. The electronic spectrum of the benzene molecule is used as an illustration. Using a (C, 4s3p2d/H, 3s2p) atomic natural orbital basis set. the following excitation energies are obtained (experimental values in parentheses): B-1(2u), 4.70 (4.90); B-1(1u), 6.10 (6.20); 1E1u, 7.06 (6.94), 1E2g, 7.77 (7.80) eV. The computed oscillator strength for the 1E1u state is 1.05 as compared to the experimental 1.25. Results of similar accuracy are obtained for the triplet states.