Pair binding in small Hubbard-model molecules

We calculate exact ground-state energies for small Hubbard-model clusters (molecules) and show that two electrons added to a molecule can experience an effective attraction despite their bare repulsive interaction (pair binding). This phenomenon takes place IonlyP for intermediate strength of the repulsive interactions, and on intermediate length scales. All the results are in qualitative agreement with those obtained using second-order perturbation theory up to surprisingly large values of the interactions and provide support for a purely electronic mechanism of superconductivity in doped ${\mathrm{C}}_{60}$.