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Resumo: A well-known property of aromatic molecules is their highly anisotropic response to an external magnetic field. This intriguing phenomenon is rationalized as a consequence of the delocalization of the itinerant electrons that populate the aromatic ring.

In this presentation, we revisit the magnetism of aromatic molecules through the study of simple Hubbard – rings, and argue that if the itinerant electrons are described by an extended Hubbard Hamiltonian with an effective momentum – momentum interaction between them, a large enhancement of the molecule diamagnetic response takes place.

We show that the presence of this new term is due to the reincorporation of part of the effects of the localized bonding electrons on the dynamics of their itinerant counterparts in Hubbard – like Hamiltonians. Going beyond the adiabatic approximation, we show that the net effect of virtual transitions of bonding electrons between their ground and excited states is to furnish the itinerant electrons with an effective interelectronic momentum – momentum interaction.

Although we have applied these ideas to the specific case of rings, our assumptions can be generalized to higher-dimensional systems sharing the required properties of which we have made use herein.