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Free energy calculations involving NH4+ in water

The analysis of the hydration of NH4+ and the estimation of relative or absolute free energies of hydration by means of Monte Carlo computer simulations using different 1‐6‐12 potential functions is reported. Two electrostatic representations of NH4+ (used respectively by W.L. Jorgensen and P.A. Kollman) in conjunction with two common water models (TIP3P and TIP4P) are considered. A change in relative hydration free energies of 1.7 kcal/mol is found when the NH4+ models are mutated into each other in either TIP3P or TIP4P. The NH4+ → Na+ mutation in both solvent models leads to similar but overestimated relative hydration energies of about −28.7 kcal/mol. Similarly, the NH4+ annihilation significantly overestimates the absolute free energy of hydration.