Improving molecular dynamics calculation of diffusivity in liquids with theoretical models

MA Orekhov, JOURNAL OF MOLECULAR LIQUIDS, 322, 114554 (2021).

DOI: 10.1016/j.molliq.2020.114554

An effect of the collective motions on the variance of diffusivity in liquids is considered. It is shown theoretically and with simulation that collective fluctuations increase statistical uncertainty of diffusivity in liquids. On this basis we develop approach that combines molecular dynamics and theoretical models to calculate diffusion coefficient in liquids. We decompose diffusivity into a low scale molecular part and a large scale hydrodynamic part. The low scale term is calculated with the molecular dynamics. It is performed using a small simulation box with less than 100 particles. Local molecular motions appear in such simulations while collective hydrodynamical fluctuations are absent The large scale hydrodynamics is included with a theoretical model. Thus, molecular dynamics term includes only molecular processes such as rearrangements of molecules, and hydrodynamical term includes collective processes. This approach provides lower statistical uncertainty compared with simulations of large systems. The improvement in statistical accuracy is shown theoretically and confirmed with simulations. It is connected to the relation between diffusivity due to local molecular processes and diffusivity due to hydrodynamics. To illustrate the application of this approach. we compute the self- diffusion coefficient in Lennard-Jones liquid and liquid water, diffusivity of divalent ions in aqueous solutions. (C) 2020 Elsevier B.V. All rights reserved.

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