CNDO/2-CI calculations on aromatic hydrocarbons

The CNDO/2 scheme is used to calculate the ground state wavefunctions for benzene, naphthalene, azulene, biphenylene, anthracene, phenanthrene, 1, 8 dimethylnaphthalene and 2, 3 dimethylnaphthalene. The core and resonance integrals are optimized to obtain good ground state and excited state properties. As a result the pi core integrals are decreased in value and the sigma core integrals are increased in value. It was also necessary to handle the resonance integrals for the sigma and pi MO's separately. The resonance integrals are multiplied by a multiplicative constant k, the constant k is 1.0 for sigma MO's and 0.476 for pi MO's. This gives values of 21 and 10 ev for the sigma and pi resonance integrals for carbon. The dipole moments, ionization potentials and charge densities are calculated and compared with the experimental values. The spectroscopic states are generated by configuration interaction within the CNDO/2 basis set. Only single-electron excitations are considered. The four lowest singlet and triplet πàπ* excited states for each molecule are compared with current experimental values. The three lowest σ, π singlet states for each molecule are also reported. Spin-orbit coupling perturbation on the configurational wavefunction is used to calculate non-zero triplet-singlet transition moments. Only the one-electron spin-orbit coupling Hamiltonian is considered. Both perturbations to the ground state wavefunction by higher triplet σ, π states and to the lowest triplet by higher σ, π singlet states are considered.