3-MOLECULAR-ORBITAL TREATMENT OF THE ORIENTATIONAL ORDERING IN A3C60

We have analyzed the band energy of the t1u band of A3C60, using the t hree-molecular-orbital tight-binding model. It turns out that this ene rgy can with a good accuracy be described by the antiferromagnetic Isi ng model, where the two allowed orientations of each molecule play the role of the two spin directions. We also show that such a description is limited to disordered and short-range ordered structures, while th e long-range ordered structures are affected by band-structure effects leading to peaks in the density of states.