Off-diagonal interactions, Hund's rules, and pair binding in C60

We have studied the effect of including nearest-neighbor electron-electron interactions, in particular the off-diagonal (non-density-density) terms, on the spectra of truncated tetrahedral and icosahedral Hubbard molecules, focusing on the relevance of these systems to the physics of doped C60. Our perturbation theoretic and exact diagonalization results agree with previous work in that the density-density term suppresses pair binding. However, we find that for the parameter values of interest for C60, the off-diagonal terms enhance pair binding, though not enough to offset the suppression due to the density-density term. We also find that the critical interaction strengths for the Hund's rules violating level crossings in C60-2, C60-3, and C60-4 are quite insensitive to the inclusion of these additional interactions