We present a model potential for studying M(q+)(H(2)O)(n=1,9) clusters where M stands for either Na(+), Cs(+), Ca(2+), Ba(2+), or La(3+). The potential energy surfaces (PES) are explored by the Monte Carlo growth method. The results for the most significant equilibrium structures of the PES as well as for energetics are favorably compared to the best ab initio calculations found in the literature and to experimental results. Most of these complexes have a different coordination number in cluster compared to experimental results in solution or solid phase. An interpretation of the coordination number in clusters is given. In order to well describe the transition between the first hydration sphere and the second one we show that an autocoherent treatment of the electric field is necessary to correctly deal with polarization effects. We also explore the influence of the cation properties (charge, size, and polarizability) on both structures and coordination number in clusters, as well as the meaning of the second hydration sphere. Such an approach shows that the leading term in the interaction energy for a molecule in the second hydration sphere is an electrostatic attraction to the cation and not a hydrogen bond with the water molecules in the first hydration sphere.

Molecular dynamics calculations were performed to simulate the collisions between a helium atom and either a water dimer or various geometries of the Na(H2O)3 + cluster. The trajectory calculations were interrogated to document the partial conversion of the collision energy into internal excitation of the cluster. Owing to the small size of helium, the collision energy, which is transferred as an impulse to the cluster, is deposited initially on one of the atoms of the cluster. The amount of transferred energy in the atom that is collided depends of its mass, and more interestingly if it is involved in a H-bond like bonding with a water molecule. The general rules that have been drawn to describe the energy transfer allowed us for a picewise construction of collision-induced-dissociation cross sections, each piece being the energy tranferred toward a specific atom of the cluster. This offers a framework for extracting quantitative information on binding energies from collision-induced-dissociation experiments by helium in M(H2O) n + systems (M is a metal atom). Importantly, the fit to the experimental data that is allowed by the present model is not restricted to the threshold energy region of the CID cross section. An application is given for the He + Au(H2O)1,2 + collision.