A comment on the application of Hartree-Fock perturbation theory to pi-electron systems

Perturbation methods of semi-empirical pi-electron theory are related to those of ab initio calculations. Three different procedures are discussed which are the analogues of (i) Hartree-Fock theory plus correlation corrections, (ii) the coupled Hartree-Fock procedure and (iii) a particular form of variational-perturbation theory. It is found that (i) and (ii) will give very similar results but that (iii) leads to ambiguous results. As an illustration, calculations are made on the pi-electron contribution to the electric polarizability of ethylene and benzene and these calculations amply confirm the theoretical expectations.