Further extension of the Madrid-2019 force field: Parametrization of nitrate (NO3-) and ammonium (NH4+) ions.

The importance of nitrate and ammonium salts both in the environment and in biological processes cannot be questioned. In this work, using the TIP4P/2005 water model, aqueous solutions of nitrate and ammonium electrolytes are parametrized using scaled charges while keeping a rigid structure and nonpolarizable charge distributions. The models are optimized by systematically testing a set of properties for twelve electrolytes-eight nitrate and four ammonium salts-thus, enlarging the number of potential chemical species encompassed within the Madrid-2019 force field for ions. The capacity of the force field for predicting densities, ion-ion and ion-solvent structures, and transport properties of the solutions comprised by the trial batch of salts was tested and discussed. Both the dependence of the densities with the salt concentration and the solution structure were nicely reproduced by the models in the whole concentration range without any trace of precipitating events and with improved accuracy in comparison with recently reported models, while the agreement of the simulated transport properties with experimental data ranges from good to reasonable, depending on the ion/counterion pair. These scaled charge models might be considered as force fields embodying a reasonable compromise between exactness and general applicability and also as an important step in the development of accurate models for polyatomic ions.