MRD‐CI studies of vertical excitation energies of unsaturated hydrocarbon molecules

Abstract

AbstractSystematic MRD‐CI calculations using the AM1 Hamiltonian have been carried out for two polyenes and eight aromatic hydrocarbons ranging from benzene to ovalene (C32H14). Twenty singlet–singlet excitation energies in these compounds were calculated and compared with experimental data and ab initio STO‐3G results. On an absolute scale, the AM1/MRD‐CI approach underestimates the excitation energies to states with dominant covalent character by an average of 1.1 eV, whereas the errors for ionic states are between −1.0 and 1.0 eV. The STO‐3G calculated data are much too high by ≈ 1 eV and ≈ 5 eV, respectively. The inclusion of σπ‐correlation effects through second‐order Epstein–Nesbet perturbation theory combined with the use of localized orbitals leads to a significant improvement of the ab initio calculated state energies. In an analogous AM1 treatment, negligible corrections for the σπ correlations are found, which is attributed to the implicit account in the parameters and approximation of the semiempirical Hamiltonian. The possible error sources of the calculational methods are discussed. © 1994 by John Wiley & Sons, Inc.