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

摘要

We studied the solvation structures of the divalent metal cations Mg~(2+)and Ca~(2+)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 Mg~(2+)is represented by a rather stable sixfold coordination in the octahedral arrangement,in agreement with experiments.Conversely,for Ca~(2+)the free-energy curve shows several shallow local minima,suggesting that the hydration structure of Ca~(2+)is highly variable.Implications for water exchange reactions are also discussed.