Scaling Solute-Solvent Distances to Improve Solubility and Ion Paring Predictions in Rigid Ion Models

Force fields based on the rigid ion model (RIM) have been developed to accurately predict the various physical and chemical properties of salts and water. However, the combined use of these models often fails to accurately predict the solubility of salts in water. To address this issue, several approaches, such as charge scaling or reparameterization, have been proposed. Nevertheless, these methods require laborious reparameterization of nonbonded force field parameters. In this article, we propose a scaling solute-solvent distance (SSSD) method to improve force fields in predicting salt solubility without changing the solute-solute and solvent-solvent interactions in the original force fields. This method can also tune the ion pairing of salt in water. One main advantage of the SSSD method is that reparameterization of the crystal and water models is not needed. We use two RIMs for the NaCl-water system (JC-SPC/E and SD-SPC/E) and the CHARMM force field for the KCl-water system to demonstrate the improved accuracy in predicting solubility by the SSSD method. Furthermore, we use the RDG-SPC/Fw force field to show that the SSSD method can also be used to tune the ion pairing of CaCO₃ in water. Limitations of this method are also discussed.