Octa coordination and the hydrated Ba2+ ion.

The hydration structure of Ba^{2+} ion is important for understanding blocking mechanisms in potassium ion channels. Here, we combine statistical mechanical theory, ab initio molecular dynamics simulations, and electronic structure methods to calculate the hydration free energy and local hydration structure of Ba^{2+}(aq). The predicted hydration free energy (-302.9$\pm$0.7 kcal/mol) matches the experimental value (-302.56 kcal/mol) when the fully occupied and exclusive inner solvation shell is treated. In the local environment defined by the inner and first shell of hydrating waters, Ba^{2+} is directly coordinated by eight (8) waters. Octa-coordination resembles the structure of Ba^{2+} and K^+ bound in potassium ion channels, but differs from the local hydration structure of K^+(aq) determined earlier.