  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.
