Investigating the Correlated Dynamics of Aqueous Solutions through Van Hove Functions: Insights from Molecular Simulation

The physical properties of water and aqueous solutions are of fundamental importance in innumerable fields. For several decades, multiple molecular simulation methods have guided experimental advances by describing the microscopic behavior of molecules and how such interactions affect macroscopic properties. As a result, it is of continued interest to evaluate the accuracy of various molecular simulation methods in describing these macroscopic behaviors in detail.

To fully understand the relationship between microscopic dynamics and macroscopic properties, such as viscosity, the collective and correlated dynamics of water must be evaluated. However, atomic correlations of fluids are often studied as static structures computed via ensemble averages. Recent advances in neutron and X-ray scattering experiments¹ have made it feasible to study the Van Hove correlation function of water and aqueous solutions². These advances have enabled the study of the time-dependent structure of these fluids at very fine resolutions, on the order of less than a picosecond in time and less than an angstrom in distance. The signal produced from these experiments however, is a statistical average over all atoms of a system, making it difficult to determine which atomic interactions most strongly drive molecular behavior. Therefore, molecular simulation is an invaluable method alongside scattering experiments to evaluate the time-dependent behavior, as the total Van Hove correlation can be discretized into separate atomic components³. Here we present Van Hove correlation results aimed at better understanding the robustness of different force fields and molecular simulation techniques describing water and aqueous solutions. Evaluated and compared to experiment in this study are the Van Hove correlation functions computed from a range of classical, polarizable, and reactive force fields, as well as ab initio molecular dynamics simulations and density functional tight binding.

[1] Iwashita, T., Wu, B., Chen, W.-R., Tsutsui, S., Baron, A. Q. R., & Egami, T. (2017). Seeing real-space dynamics of liquid water through inelastic x-ray scattering. Science Advances, 3(12), e1603079. https://doi.org/10.1126/sciadv.1603079

[2] Van Hove, L. (1954). Correlations in space and time and born approximation scattering in systems of interacting particles. Physical Review, 95(1), 249–262. https://doi.org/10.1103/PhysRev.95.249

[3] Shinohara, Y., Matsumoto, R., Thompson, M., Dmowski, W., Ryu, C. W., Iwashita, T., Ishikawa, D., Baron, A., Cummings, P., Egami, T. (2019). Identifying Water-Anion Correlated Motions in Aqueous Solutions through Van Hove Functions. Journal of Physical Chemistry Letters, 10(22), 7119-7125. https://doi.org/10.1021/acs.jpclett.9b02891