Anion polarizability in solution: does it depend on the environment?

It is widely accepted that the inclusion of polarization effects in force field Molecular Dynamics (MD) simulations is of high importance for studying both homogeneous and inhomogeneous systems, and that a proper description of polarizable interactions represents the next milestone in force field development [1]. In recent years we have studied a simple system, chloride in water, to face the problem of describing electrostatic interactions at short range [2-4]. We have found that both at gas and at condensed phase, the best results for static and dynamical properties are obtained if damping functions are used together with gas phase polarizability for the anion [2,5,6]. This result is certainly surprising if one considers that all simulations at condesed phase account for a reduced polarizability of the anion. In our recent review we have compared classical MD with ab initio results, confirming that our approach is correct. Here we pose a subtle question about the physical interpretation of the above result, address- ing it with ab initio MD of chloride in water. The conclusions, of general validity for small ions in water solution, constitute a theoretical framework for future developments in the field [7-9].
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