COMPLETE ACCOUNT OF THE PI-ELECTRON CORRELATION IN CALCULATING RING CURRENTS IN CONJUGATED AROMATIC AND ANTIAROMATIC SYSTEMS

A semiempirical theory of ring pi currents is developed which accurate ly fakes into account the electron correlation using the full configur ation interaction (FCI) method An elementary model is proposed in whic h the calculation of the pi-electron correlation is reduced to the mod el of generalized alternative Huckel systems. The FCI and SCF (self-co nsistent field) methods are used to calculate the diamagnetic suscepti bilities and ring currents for some typical pi systems. It is shown th at as a rule, the SCF method allows the qualitatively true description of the magnetic properties of condensed systems, although for antiaro matic molecules if often yields underestimated values of paramagnetic currents. The results of the FCI calculations suggest that electronic excitations can induce transitions from pi paramagnetism to pi diamagn etism (e.g., in fulvenes) and back (in quinones).