Ab initio approach to superexchange interactions in alkali doped fullerides AC₆₀

The superexchange interactions between the fullerenes arise as a result of the electron transfer from the C₆₀ molecule to the alkali atom and back. We present a scheme, which is a configuration interaction approach based on the valence bond (Heitler‐London) method. The effect of superexchange is described together with chemical bonding by constructing and solving a secular equation, rather than by using a perturbation treatment. We have considered 180° and 90° superexchange for the C₆₀‐Cs‐C₆₀ pathways. The calculations account for unusual electronic properties of polymer orthorhombic and quenched cubic phases of CsC₆₀: two lines in nuclear magnetic resonance experiments, the development of a spin‐singlet ground state and a decrease of magnetic susceptibility as T→0.