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J Chem Phys. 2007 Aug 21;127(7):074503.

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

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Quantum Beam Science Directorate, Japan Atomic Energy Agency (JAEA), 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan.


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.

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