Interactions of theNa2dimer with aC60molecule

The interactions of the ${\mathrm{Na}}_2$ dimer with a ${\mathrm{C}}_{60}$ molecule are examined by considering endohedral and exohedral complexes. Five different sites for the adsorbate are investigated: fivefold, threefold, long bond, short bond, and on top. Two orientations of the dimer with respect to the symmetry axes of the fullerene, parallel and perpendicular, were considered. The ground-state electronic configuration for both the endohedral ${\mathrm{C}}_{60}$ ${\mathrm{Na}}_2$ and the exohedral ${\mathrm{C}}_{60}$ ${\mathrm{Na}}_2$ complexes is found to be a triplet. The largest adsorption energy was found for the on-top (T) adsorption in the endohedral ${\mathrm{C}}_{60}$ ${\mathrm{Na}}_2$ complex. All the endohedral complexes were found to be stable against fragmentation into the three important dissociation channels, including the dissociation into a Na atom and a ${\mathrm{C}}_{60}$Na fragment. The relative stabilities of the different complexes were related to their structural parameters. For exohedral ${\mathrm{C}}_{60}$ ${\mathrm{Na}}_2$, the triplet electronic configuration destabilizes and the singlet state becomes dominant for adsorbate distances larger than 6 Å. The highest occupied molecular orbital–lowest unoccupied molecular orbital gap in both the endohedral and exohedral complexes was found to be approximately half of its value in the bare fullerene. Trends in the electronic energy levels of the two types of complexes are discussed in detail. © 1996 The American Physical Society.