Nucleation of a sodium droplet onC60

We investigate theoretically the progressive coating of ${\mathrm{C}}_{60}$ by several sodium atoms. Density functional calculations using a nonlocal functional are performed for ${\mathrm{NaC}}_{60}$ and ${\mathrm{Na}}_2{\mathrm{C}}_{60}$ in various configurations. These data are used to construct an empirical atomistic model in order to treat larger sizes in a statistical and dynamical context. Fluctuating charges are incorporated to account for charge transfer between sodium and carbon atoms. By performing systematic global optimization in the size range $1<~n<~30,$ we find that ${\mathrm{Na}}_n{\mathrm{C}}_{60}$ is homogeneously coated at small sizes, and that a growing droplet is formed above $n>~8.$ The separate effects of single ionization and thermalization are also considered, as well as the changes due to a strong external electric field. The present results are discussed in the light of various experimental data.