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arXiv 0912.4526physics.chem-phphysics.comp-ph
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基于单离子和离子对溶剂化性质的离子力场优化

Ionic force field optimization based on single-ion and ion-pair solvation properties

Maria Fyta, Immanuel Kalcher, Joachim Dzubiella, Lubos Vrbka, Roland R. Netz

摘要:

离子溶液的分子动力学模拟敏感地依赖于用于离子的力场。为了解决相同化合价和大致相似大小的离子之间的细微差异,特别是为了正确描述离子特异性效应,很明显,精确的力场是必要的。在过去,离子力场的优化策略要么考虑单离子性质(如无限稀释时的溶剂化自由能或离子-水结构),要么考虑离子对性质(以离子-离子分布函数的形式)。在本文中,我们研究的策略,以优化离子力场的基础上,单离子和离子对的属性同时。为此,我们模拟了五种不同的盐溶液,即CsCl,KCl,NaI,KF和CsI,在有限的离子浓度。在单离子溶剂化自由能与实验值相匹配的约束下,这些离子的力场系统地变化,这将Lennard Jones相互作用的二维$\{σ,ε\}$参数空间减少到每个离子的一维线。从有限浓度模拟,提取的对势和渗透系数的计算,这是比较实验数据。我们发现一个很强的依赖性的渗透系数的力场,这是显着的单离子溶剂化自由能和离子-水结构保持不变的参数变化下。对于阳离子Cs $^+$和K $^+$,实现了力场的优化,而对于阴离子I $^-$和F $^-$,不能达到实验渗透系数。这表明,从长远来看,可能必须在建模中引入额外的参数,例如通过修改混合规则。

英文摘要:

Molecular dynamics simulations of ionic solutions depend sensitively on the force fields employed for the ions. To resolve the fine differences between ions of the same valence and roughly similar size and in particular to correctly describe ion-specific effects, it is clear that accurate force fields are necessary. In the past, optimization strategies for ionic force fields either considered single-ion properties (such as the solvation free energy at infinite dilution or the ion-water structure) or ion-pair properties (in the form of ion-ion distribution functions). In this paper we investigate strategies to optimize ionic force fields based on single-ion and ion-pair properties simultaneously. To that end, we simulate five different salt solutions, namely CsCl, KCl, NaI, KF, and CsI, at finite ion concentration. The force fields of these ions are systematically varied under the constraint that the single-ion solvation free energy matches the experimental value, which reduces the two-dimensional $\{σ,ε\}$ parameter space of the Lennard Jones interaction to a one dimensional line for each ion. From the finite-concentration simulations, the pair-potential is extracted and the osmotic coefficient is calculated, which is compared to experimental data. We find a strong dependence of the osmotic coefficient on the force field, which is remarkable as the single-ion solvation free energy and the ion-water structure remain invariant under the parameter variation. Optimization of the force field is achieved for the cations Cs$^+$ and K$^+$, while for the anions I$^-$ and F$^-$ the experimental osmotic coefficient cannot be reached. This suggests that in the long run, additional parameters might have to be introduced into the modeling, for example by modified mixing rules.

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