In our ABFE thermodynamic cycle, we chose the harmonic restraint on the trapped water to be weak enough that the restraint does not significantly disturb the dynamics of the system when the water is interacting, but strong enough that the weakly interacting or noninteracting water does not stray too far from the binding site. For this reason, we restrained the trapped water at its target binding site using a harmonic restraint with a light force constant of 0.5 kcal mol -1 Å -2 .