Optimum parametrization in SCF π-electron theory

An attempt has been made to obtain ground-state charge distributions (as reflected by proton and carbon-13 magnetic shielding constants), ionization potentials, and pi-electron excitation energies for a number of substituted benzenes. A standard SCF-LCAO-MO method was employed, with the total-spin-differential overlap approximation. Excitation energies were obtained by the method of configuration interaction using all singly-excited configurations. Approximations were introduced so that only two parameters had to be specified for each atom. Since the two-centre repulsion integrals introduced by Mataga gave the best results for benzene, they were used throughout. Heteroatom parameters were chosen empirically, with the help of McWeeny's self-consistent perturbation theory, to give the best overall results for monosubstituted compounds. In general the calculated results were very reasonable.