Acidifying the Madrid-2019 force field: A rigid model for H3O+ with scaled charges

A classical and rigid force field for the oxonium cation, H3O+, optimized in solutions of TIP4P/2005 water, is introduced. While the charges of both H3O+ and the selected counteranions (i.e., Cl-, Br-, I-, and NO3-) are scaled by a factor of 0.85, following the philosophy of the so-called Madrid-2019 model for ions, the charge distribution of H3O+ was derived within the framework of the self-consistent atomic dipole-corrected Hirshfeld approach. Considering the simplicity of the model, the agreement between experimental data and molecular dynamics simulation results for the curvature of the solution density as a function of the solute concentration is remarkable. However, limitations persist in capturing ion-pairing behavior and long-range hydrogen-bonding dynamics in polyatomic systems. We found that a scaled charge of 0.85e provides an accurate description of the local structure of hydrogen halides but is detrimental to predicting the viscosity of the solution. The opposite effect is observed for HNO3. Nonetheless, the newly optimized potential parameters for H3O+ expand the family of ions with scaled charges in the Madrid-2019 force field, providing a computationally efficient and versatile platform to study electrolyte solutions in acidic environments. These findings contribute to the advancement of molecular modeling techniques and to improving our understanding of the interplay between local structure (solvation, ion pairing) and transport properties in complex systems.