CORRELATION-EFFECTS ON ELECTRONIC AND OPTICAL-PROPERTIES OF A C-60 MOLECULE - A VARIATIONAL MONTE-CARLO STUDY

The electronic and optical properties of the neutral C-60 molecule are investigated in the extended Su-Schrieffer-Heeger model including a H ubbard-type on-site interaction by the variational Monte Carlo (VMC) m ethod. The optical energy gap E(g) of the molecule and the energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupi ed molecular orbital (LUMO) have been calculated as functions of the H ubbard interaction strength U divided by the hopping constant t. It is found that the energy of both the HOMO and LUMO levels increase almos t 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 polym ers. Pair-binding energies in the singlet and triplet states have also been calculated by the VMC method for nondimerized molecules, and a c omparison has been made with the results obtained by perturbation theo ry.