Coulomb and exchange interactions in C60n-

Abstract

Semiempirical modified-neglect-of-differential-overlap (MNDO) quantum chemistry calculations are performed for the negative ions of an isolated ${\mathrm{C}}_{60}$. 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 ${\mathrm{C}}_{60}^{2\mathrm{\ensuremath{-}}}$ and ${\mathrm{C}}_{60}^{3\mathrm{\ensuremath{-}}}$ allow us to extract an effective on-site repulsion ${\mathit{U}}_{0}$\ensuremath{\sim}3 eV, and a direct exchange interaction K\ensuremath{\sim}50 meV for ${\mathrm{C}}_{60}$. These values are used to estimate the effective on-site repulsion and dielectric constant of solid ${\mathrm{C}}_{60}$, with results in good agreement with experiment. A similar approach is used to determine ``partially'' screened parameters appropriate to a model of ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$. We find that both the on-site (${\mathit{U}}_{0}$\ensuremath{\sim}1.3 eV) and near-neighbor (V\ensuremath{\sim}0.3 eV) Coulomb terms are significant relative to the bandwidth (W\ensuremath{\sim}0.6 eV).