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.