Coulomb couplings in positively charged fullerene

Abstract

We compute, based on density-functional electronic-structure calculations, the Coulomb couplings in the h_u highest occupied orbital of molecular C60. We obtain a multiplet-averaged Hubbard U ~ 3 eV, and four Hund-rule-like intra-molecular multiplet-splitting terms, each of the order of few hundreds of meVs. According to these couplings, all C60^n+ ions should possess a high-spin ground state if kept in their rigid, undistorted form. Even after molecular distortions are allowed, however, the Coulomb terms still appear to be somewhat stronger than the previously calculated Jahn-Teller couplings, the latter favoring low-spin states. Thus for example in C60^2+, unlike C60^2-, the balance between Hund rule and Jahn Teller yields, even if marginally, a high-spin ground state. That seems surprising in view of reports of superconductivity in field-doped C60^n+ systems.