Theoretical study of the binding of Na clusters encapsulated in the C₂₄₀ fullerene

Density functional theory has been used to study the electronic structure and binding of Na_N clusters (N≤30) encapsulated inside the large fullerene C₂₄₀. 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. © 1996 The American Physical Society.