Polarization and charge-transfer contributions have been shown to be nonnegligible in the binding energy of [UO2(H2O)]2+ and have been quantified by Coordination Energy Partitioning. In this context, two types of model potential for uranyl cation-water clusters, i.e., one including polarization effects explicitly and charge-transfer effects implicitly, and the other including these two effects explicitly, have been derived from ab initio calculations on [UO2(H2O)]2+. Only the model with the explicit charge-transfer term reproduces accurately the ab initio geometries and energies of small clusters containing up to five water molecules. An exploration of the potential energy surface of clusters with up to eight water molecules has been performed with the Monte Carlo growth method. The first coordination sphere in clusters contains five water molecules, as in experimental liquid and solid phases. The presence of further water molecules in the second and third coordination spheres reinforces the preference for five in the first shell. The coordination number five results from a subtle competition between polarization and repulsion contributions on one hand, which favor molecules in the second coordination sphere, and electrostatic and charge-transfer contributions on the other hand, which obviously favor molecules in the first coordination sphere. In addition, the water molecules in the second coordination sphere are principally oriented by a strong electrostatic interaction with the uranyl cation, but also interact weakly with two water molecules of the first coordination sphere.
