Construction of a Polarizable Force Field for Molecular Dynamics Simulation of a NaOTF Water-In-Salt Electrolyte

To model a NaOTF Water-in-Salt (WiS) electrolyte using classical Molecular Dynamics (MD) simulations, we explore various force fields where atomic polarization is accounted for at three different levels: a non-polarizable all-atom force field where polarization is only implicitly included in its Van der Waals interaction parameters, the same force field with uniformly scaled ionic charges mimicing electron polarization within a mean-field approximation, and an explicit polarizable force field where polarization is modeled via Drude oscillators. We also probe combining different polarization levels for salt ions and water: when ion polarization is described by the Drude method, water is modeled by either the non-polarizable SPC/E model or the polarizable SWM4-NDP model. The main goal is to achieve simulation stability for different force fields and investigate the influence of the force field parameters on the electrolyte properties. Force field parameters that adjust the interactions of cations or Drude pairs are found to significantly affect the electrolyte structure and its dynamic properties. This effect is primarily due to the strong dependence of the degree of salt dissociation on these parameters. Among the force fields studied in this work, we identify an efficient combination of the Drude polarizable force field with non-polarizable water models, which is sufficiently flexible to reproduce various properties of WiS solutions, while the computational cost is affordable and simulation stability is ensured over a relatively wide range of force field parameters.