The role of charge distribution in the modeling of polyatomic ions: The nitrate anion case

Nitrate salts are widely used in agriculture, industrial chemistry, and food preservation. Using computer simulations based on the intermolecular parameters of the extended Madrid-2019 force field [V. M. Trejos et al., J. Chem. Phys. 159(22), 224501 (2023)], we examine the effect of intramolecular partial charge distributions of the nitrate anion on the temperature of maximum density (TMD) of aqueous NaNO3, KNO3, and NH4NO3 solutions. A specific internal charge distribution improves agreement with experimental TMD data relative to the original force field and enables predictions for systems lacking experimental measurements. Densities are largely insensitive to the charge distribution, while transport properties (diffusion coefficients and viscosities) show only minor variations. In contrast, structural properties are sensitive to the choice of the charge distribution. The original force field provides better agreement with neutron diffraction structural data than the model yielding the most accurate TMD. An intermediate charge distribution is therefore selected as a compromise, providing a balanced description of all properties. Empirical force fields cannot reproduce all properties simultaneously; however, an appropriate parameter selection, including the intramolecular charge distribution as an additional degree of freedom, yields a consistent description of thermodynamic, transport, and structural properties.