Hydration properties of magnesium and calcium ions from constrained first principles molecular dynamics

We studied the solvation structures of the divalent metal cations Mg2+ and Ca2+ in ambient water by applying a Car-Parrinello-based constrained molecular dynamics method. By employing the metal-water oxygen coordination number as a reaction coordinate, we could identify distinct aqua complexes characterized by structural variations of the first coordination shell. In particular, our estimated free-energy profile clearly shows that the global minimum for Mg2+ is represented by a rather stable sixfold coordination in the octahedral arrangement, in agreement with experiments. Conversely, for Ca2+ the free-energy curve shows several shallow local minima, suggesting that the hydration structure of Ca2+ is highly variable. Implications for water exchange reactions are also discussed.