The determination of single ion hydration free energies is troubled by the thermodynamic constraint that only the properties of neutral pairs can be uniquely determined. As such, single ion properties depend on extrathermodynamic information, which can differ between experimental and molecular simulation measurements. This comparison is hampered by the quantum mechanical nature of the proton, the reference ion of choice for developing standard tables, and uncertainty in the experimental reference potential to which properties are measured. We revisit the methodology of Latimer et al. [J. Chem. Phys. 7, 108 (1939)], which extracts single ion properties from neutral pair transfer free energies under the assumption that the Born equation provides an accurate description of the charging of monovalent ions. This methodology permits us to make a consistent comparison between experimental and theoretical values for single ion hydration free energies and gives insight into nonpolar contributions to the ion hydration free energy as well as the potential at the center of a hypothetical uncharged ion.
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The constant is not explicitly written in the Born equation given in Ref. 9. Rather, Latimer et al. described a third constant required to divide the pair free energy above and beyond and . This constant is determined so the fits for the positive and negative ions fall on the same theoretical curve (Fig. 2 in Ref. 9). The effect of this division is the addition of a constant to the single ion hydration free energies.
