VARIATIONAL MONTE-CARLO AND CONFIGURATION-INTERACTION STUDIES OF C-60AND ITS FRAGMENTS

The C-60 molecule and its fragments are studied using configuration in teraction and variational Monte Carlo techniques, within the Hubbard m odel. Using benzene as a test case, we compare the results of the appr oximate calculations with exact calculations. The fragments of C-60 st udied are pyracylene, fluoranthene, and corannulene. The energies, bon d orders, spin-spin, and charge-correlation functions of these systems are obtained for various values of the Hubbard parameter, U. The anal yses of bond orders and correlation functions of these individual mole cules allow us to visualize pyracylene as a naphthalene unit with two ethylenic moieties and fluoranthene as weakly bridged benzene and naph thalene units. Corannulene is the largest fragment of C-60 that we hav e studied. The hexagon-hexagon (h-h) bond orders are slightly larger t han those of the hexagon-pentagon (h-p) bonds, a feature also found in other fragments. We also find bonds between two coordinated carbon si tes to be stronger than bonds involving three coordinated carbon sites . In C-60, the h-h bonds are stronger than in corannulene and the h-p bonds weaker than in corannulene for all correlation strengths. Introd ucing bond alternation in the buckyball enhances this difference.