The electronic and optical properties of the neutral C-60 molecule are investigated in the extended Su-Schrieffer-Heeger model including a Hubbard-type on-site interaction by the variational Monte Carlo (VMC) method. The optical energy gap E(g) of the molecule and the energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) have been calculated as functions of the Hubbard interaction strength U divided by the hopping constant t. It is found that the energy of both the HOMO and LUMO levels increase almost equally with increase of Ult, so that the Hubbard term Ult has only a weak effect on E(g) for intermediate interaction strengths (Ult<5). This is significantly different from the situation in conducting polymers. Pair-binding energies in the singlet and triplet states have also been calculated by the VMC method for nondimerized molecules, and a comparison has been made with the results obtained by perturbation theory.
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