THEORETICAL-STUDY OF THE BINDING OF NA CLUSTERS ENCAPSULATED IN THE C-240 FULLERENE

Density functional theory has been used to study the electronic struct ure and binding of Na-N clusters (N less than or equal to 30) encapsul ated inside the large fullerene C-240. One or more electrons are alway s transferred from the endohedral Na-N to the cage. The charge density distribution reveals that for small N the binding between Na-N and th e fullerene is purely ionic, and that for N>9 a covalent contribution to the bonding develops, enhanced by the expansion of the endohedral N a-N. The evolution of the type of bonding with increasing N is analyze d by comparing the size variations of the binding energy and the ioniz ation potential of the Na-N clusters.