To model the adsorption of Na
+ in aqueous solution on the semiconductor surface, the interactions of Na
+and Na
+(H
2O)
n (
n = 1-6) with a clean Si(111) surface were investigated by using hybrid density functionaltheory (B3LYP) and M
ller-Plesset second-order perturbation (MP2) methods. The Si(111) surface wasdescribed with Si
8H
12, Si
16H
20, and Si
22H
21 cluster models. The effect of the basis set superposition error(BSSE) was taken into account by applying the counterpoise (CP) correction. The calculated results indicatedthat the interactions between the Na
+ cation and the dangling bonds of the Si(111) surface are primarilyelectrostatic with partial orbital interactions. The magnitude of the binding energies depends weakly on theadsorption sites and the size of the clusters. When water molecules are present, the interaction between theNa
+ and Si(111) surfaces weakens and the binding energy has the tendency to saturate. On a Si
22H
21 clusterdescribed surface, the optimized Na
+-surface distance for Na
+(H
2O)
5 adsorbed at on-top site is 4.16 Å andthe CP-corrected binding energy (MP2) is -35.4 kJ/mol, implying a weakly adsorption of hydrated Na
+cation on clean Si(111) surface.