INTERACTIONS OF THE NA-2 DIMER WITH A C-60 MOLECULE

The interactions of the Na-2 dimer with a C-60 molecule are examined b y considering endohedral and exohedral complexes. Five different sites for the adsorbate are investigated: fivefold, threefold, long bond, s hea bond, and on top. Two orientations of the dimer with respect to th e symmetry axes of the fullerene, parallel and perpendicular, were con sidered. The ground-stale electronic configuration for both the endohe dral C60Na2 and the exohedral C60Na2 complexes is found to be a triple t. The largest adsorption energy was found for the on-top (T) adsorpti on in the endohedral C60Na2 complex. All the endohedral complexes were found to be stable against fragmentation into the three important dis sociation channels, including the dissociation into a Na atom and a C6 0Na fragment. The relative stabilities of the different complexes were related to their structural parameters. For exohedral C60Na2, the tri plet electronic configuration destabilizes and the singlet state becom es dominant for adsorbate distances larger than 6 Angstrom. The highes t occupied molecular orbital-lowest unoccupied molecular orbital gap i n both the endohedral and exohedral complexes was found to be approxim ately half of its value in the bare fullerene. Trends in the electroni c energy levels of the two types of complexes are discussed in detail.