Molecular force fields for aqueous electrolytes: SPC/E-compatible charged LJ sphere models and their limitations

Thirteen of the most common aqueous NaCl solution force fields based on the SPC/E water solvent are examined with respect to their prediction at ambient conditions of the concentration dependence of the total electrolyte chemical potential and the solution density. We also calculate the salt solubility and the chemical potential and density of the NaCl crystalline solid. We obtain the solution chemical potential in a computationally efficient manner using our recently developed Osmotic Ensemble Monte Carlo method [F. Moucka, M. Lisal, and W. R. Smith, J. Phys. Chem. B 116, 5468 (2012)]. We find that the results of the force fields considered are scattered over a wide range of values, and none is capable of producing quantitatively accurate results over the entire concentration range, with only two of them deemed to be acceptable. Our results indicate that several force fields exhibit precipitation at concentrations below the experimental solubility limit, thus limiting their usefulness. This has important implications, both in general and for their use in biomolecular simulations carried out in the presence of counter-ions. We conclude that either different parameter fitting techniques taking high-concentration properties into account must be used when determining force field model parameters, or that the class of models considered here is intrinsically incapable of the task and more sophisticated mathematical forms must be used. (C) 2013 AIP Publishing LLC. [http://dx. doi. org/10.1063/1.4801322]