Coulomb couplings in positively charged fullerene

Abstract We compute, based on density-functional electronic-structure calculations, the Coulomb couplings in the hu highest occupied orbital of molecular C60. We obtain a multiplet-averaged Hubbard U ≈ 3 eV, and four Hund-rule-like intramolecular multiplet-splitting terms, each of the order of a few hundred millielectronvolts. According to these couplings, all Cn+ 60 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 favouring low-spin states. Thus for example in C2+ 60, unlike C2- 60, the balance between the 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 Cn+ 60 systems.