Ferromagnetic spin interactions between benzene anions: Molecular-orbital analysis and the Hubbard model

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Abstract
As a model for ferromagnetism in an organic system, the spin interactions between two benzene-radical anions have been considered. Ab initio molecular-orbital calcuations for the energies of this dimer show that the ground state is a triplet for all separations of the benzene molecules, indicating a ferromagnetic interaction between spins on adjacent molecules. Furthermore, a three-parameter Hubbard model gives a quantitative fit to the energies of the 12 low-lying states.