COULOMB AND EXCHANGE INTERACTIONS IN C60(N-)

Semiempirical modified-neglect-of-differential-overlap (MNDO) quantum chemistry calculations are performed for the negative ions of an isolated C60. The value computed for the first electron affinity is in excellent agreement with experiment. MNDO predictions of the energies of the low-lying states of C60(2-) and C60(3-) allow us to extract an effective on-site repulsion U0 approximately 3 eV, and a direct exchange interaction k approximately 50 meV for C60. These values are used to estimate the effective on-site repulsion and dielectric constant of solid C60, with results in good agreement with experiment. A similar approach is used to determine ''partially'' screened parameters appropriate to a model of K3C60. We find that both the on-site (U0 approximately 1.3 eV) and near-neighbor (V approximately 0.3 eV) Coulomb terms are significant relative to the bandwidth (W approximately 0.6 eV).