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Abstract: Molecular dynamics simulations of ionic solutions depend sensitively on theforce fields employed for the ions. To resolve the fine differences betweenions of the same valence and roughly similar size and in particular tocorrectly describe ion-specific effects, it is clear that accurate force fieldsare necessary. In the past, optimization strategies for ionic force fieldseither considered single-ion properties such as the solvation free energy atinfinite dilution or the ion-water structure or ion-pair properties in theform of ion-ion distribution functions. In this paper we investigatestrategies to optimize ionic force fields based on single-ion and ion-pairproperties simultaneously. To that end, we simulate five different saltsolutions, namely CsCl, KCl, NaI, KF, and CsI, at finite ion concentration. Theforce fields of these ions are systematically varied under the constraint thatthe single-ion solvation free energy matches the experimental value, whichreduces the two-dimensional $\{\sigma,\epsilon\}$ parameter space of theLennard Jones interaction to a one dimensional line for each ion. From thefinite-concentration simulations, the pair-potential is extracted and theosmotic coefficient is calculated, which is compared to experimental data. Wefind a strong dependence of the osmotic coefficient on the force field, whichis remarkable as the single-ion solvation free energy and the ion-waterstructure remain invariant under the parameter variation. Optimization of theforce field is achieved for the cations Cs$^+$ and K$^+$, while for the anionsI$^-$ and F$^-$ the experimental osmotic coefficient cannot be reached. Thissuggests that in the long run, additional parameters might have to beintroduced into the modeling, for example by modified mixing rules.



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

Fuente: https://arxiv.org/







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