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Potentials of mean force computed for the Na – Na ion pair in PME simulations. The total charge in the simulations without counterions is neutralized by uniform charge background. This has the same effect as explicit counterions fixed in space. Free counterions are seen to lead to markedly different PMFs. The simulations were performed in a L = 45 Å box. The free energy curves were made to overlap at r = 1.2 nm.
