GAUGE-INVARIANT CALCULATIONS OF MAGNETIC-PROPERTIES IN SEMIEMPIRICAL APPROACHES - APPLICATION TO FULL-CI PI-ELECTRON SCHEME

A new semiempirical modification for magnetic property calculations en suring the gauge invariance is proposed. It is based on the choice of the electronic charge centroid as a universal gauge origin and a speci al representation of the diamagnetic part of proper tensors by using s tandard commutation relations for angular moments. In the case of the pi-electron model the approach furnishes a justification of the well-k nown London approximation. Furthermore, the full-CI method for pi-elec trons is found to be automatically gauge invariant (without invoking t he electronic centroid). This allows us to prove the full-CI level of the appropriate Kirchhoff law for the ring pi-currents and realize the approach for generic systems studying, e.g. their magnetic behaviour in relation to spin multiplicity.