Determining the solvation free energies of single ions in water is one of the most fundamental problems in physical chemistry and yet many unresolved questions remain. In particular, the ability to decompose the solvation free energy into simple and intuitive contributions will have important implications for models of electrolyte solution. Here, we provide definitions of the various types of single ion solvation free energies based on different simulation protocols. We calculate solvation free energies of charged hard spheres using density functional theory interaction potentials with molecular dynamics simulation and isolate the effects of charge and cavitation, comparing to the Born (linear response) model. We show that using uncorrected Ewald summation leads to unphysical values for the single ion solvation free energy and that charging free energies for cations are approximately linear as a function of charge but that there is a small non-linearity for small anions. The charge hydration asymmetry for hard spheres, determined with quantum mechanics, is much larger than for the analogous real ions. This suggests that real ions, particularly anions, are significantly more complex than simple charged hard spheres, a commonly employed representation.

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带有密度泛函理论相互作用的分子动力学的带电硬球的静电溶剂化自由能

确定水中单离子的溶剂化自由能是物理化学中最基本的问题之一，但仍有许多未解决的问题。特别地，将溶剂化自由能分解成简单直观的贡献的能力将对电解质溶液的模型具有重要意义。在这里，我们提供了基于不同模拟协议的各种类型的单离子溶剂化自由能的定义。我们使用分子动力学模拟的密度泛函理论相互作用势来计算带电硬球的溶剂化自由能，并与Born（线性响应）模型相比，将电荷和空化的影响隔离开来。我们表明，使用未经校正的Ewald总和会导致单离子溶剂化自由能的非物理值，并且阳离子的带电自由能与电荷近似成线性关系，但是对于小阴离子而言，非线性很小。由量子力学确定的硬球的电荷水合不对称性远大于类似的真实离子。这表明，实际离子，特别是阴离子，比简单带电的硬球体（一种常用的表示形式）要复杂得多。