Theoretical study of the binding of Na clusters encapsulated in the ${\mathrm{C}}_{240}$ fullerene

Density functional theory has been used to study the electronic structure and binding of ${\mathrm{Na}}_{\mathit{N}}$ clusters (N≤30) encapsulated inside the large fullerene ${\mathrm{C}}_{240}$. One or more electrons are always transferred from the endohedral ${\mathrm{Na}}_{\mathit{N}}$ to the cage. The charge density distribution reveals that for small N the binding between ${\mathrm{Na}}_{\mathit{N}}$ 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 ${\mathrm{Na}}_{\mathit{N}}$. 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 ${\mathrm{Na}}_{\mathit{N}}$ clusters. © 1996 The American Physical Society.