On the orbital analysis of magnetic properties

A general definition of orbital contributions, valid for any single-valued distribution of origin of vector potential, is given for the current density induced in a molecule by an external magnetic field. Only for the ipsocentric choice of origin, where current density at each point is calculated with that point as origin, are the orbital contributions free of unphysical occupied-occupied mixing terms, and in this scheme, response of delocalised pi systems is restricted to the activity of a small number of frontier electrons, and governed by simple symmetry rules. Localised orbitals can equally well be used in this framework for sigma systems. The advantages of the ipsocentric method extend to the integrated properties of magnetisability and nuclear shielding, giving what is, in a well defined sense, the best orbital contributions for interpretation of these properties and their links to aromaticity. The general theory is illustrated by detailed calculations of the magnetic response properties of the benzene molecule.