Binding free energies for Na+ ions in various locations (in kcal/mol).

The interaction energy (ΔGint), entropic contributions (ΔGtr) and the total binding energy (ΔGb) are listed separately. The interaction free energy differences are obtained from the average of the forward and backward transitions. Presence of a second Na+ ion is indicated in parentheses. Errors are estimated from block data analysis using 50 ps windows. The Na2 site is not considered because it is formed only after the HP2 gate shuts.