Theoretical study of the binding of Na clusters encapsulated in the C-240 fullerene

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