Scaled-charge ion force field optimized for individual ion partial molar volumes in water.

We present an ion partial molar volume force field for common monovalent and divalent ionic species, including Li+, Na+, K+, Cs+, Mg2+, Ca2+, F-, Cl-, Br-, I-, OH-, and H3O+. When combined with the TIP4P/2005 water model, the force field enables quantitative investigations of ion-specific effects in both bulk and interfacial properties. The iPMV force field is developed based on a single guiding principle: the ion-water interaction parameters are optimized such that the experimental partial molar volume of each individual ion at infinite dilution-determined via the ultrasonic vibrational potential method-is reproduced by construction. An exception is made for the hydronium ion, for which we also provide an alternative interface-optimized model. The force field employs a charge scaling factor of 0.75 and adopts the Lorentz-Berthelot combining rules for cross-Lennard-Jones interactions, with specific modifications for divalent cations. A distinctive feature of the iPMV force field is that, despite its straightforward design, a single set of ion parameters consistently reproduces a wide range of experimental properties-from volumetric and interfacial properties to salting-out/in effects and transport coefficients. This robust performance suggests a fundamental correlation between the partial molar volumes of individual ions and the broader physical properties of aqueous electrolyte solutions.