A Multiscale Approach to Analyzing the Van Hove Correlations of Water

The physical properties of water are of fundamental importance in innumerable fields. Despite the many modes of analysis used to describe water including viscosity, self-diffusion coefficients, and pair distribution functions (PDFs), there is still much to be understood regarding water and the molecular origins of its unique properties. Recently, several studies have calculated the time-dependent pair distribution function, also known as the Van Hove function (VHF), for the first time through a combination of inelastic x-ray scattering (IXS) and classical molecular dynamics (CMD) simulations. Through these analyses, water molecules were found to be correlated in both space and time.

Although CMD simulations produce qualitative agreement with the IXS response, the chosen models lack the details required to quantitatively reproduce the interactions of water. To do so, a multiscale approach with the inclusion of quantum mechanical and reactive approaches is required. Ab initio MD (AIMD) is a quantum mechanical approach that computes electronic structure calculations on the fly to generate forces for dynamics. Though computationally expensive, these calculations allow these methods to be unbiased to a given state or set of reference data. Density-functional tight-binding (DFTB) is an approximation to density functional theory that allows for routine larger scale simulations on nanosecond time scales. Reactive MD includes additional interaction terms to account for chemical reactions and hydrogen bonding. Here we present VHF results aimed at better understanding the robustness of molecular simulation techniques describing water at a multitude of time and length scales. Encompassed in this study are a range of classical, polarizable, and reactive force fields, as well as AIMD, and DFTB.

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

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