Hydration of sodium in water clusters

Hydration of sodium in water clusters, Na(${\mathrm{H}}_2$O${)}_{\mathit{n}}$, 1≤n≤8, is investigated using nonlocal pseudopotentials and local-spin-density functional theory, with exchange-correlation gradient corrections. Addition of water molecules to an Na atom results in a successive decrease in the ionization potential, with a marked reduced variation for n>4. This reflects the formation of a molecular shell about the Na for n∼4, accompanied by expulsion of the metal valence electron from the hydration cavity and its delocalization in a ‘‘surface Rydberg-like state.’’