Abstract
The Hubbard model with two orbitals per atom in its insulating limit is shown to be equivalent to a special case of the generalized spin Hamiltonian including two orbital states, which has been put forth previously. In the light of this equivalence, it is pointed out that the recent theory of exciton line broadening in Mott-Hubbard insulators by Doniach, Roulet, and Fisher corresponds to a rather unrealistic limit of large spin-dependent excitation transfer.

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