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

Ikeda, Takashi; Boero, M.*; Terakura, Kiyoyuki*

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 six-fold 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.

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