Stefan–Maxwell diffusivities extracted from molecular dynamics simulations via Onsager’s regression hypothesis*

We report on our development of a method to measure macroscopic diffusion coefficients in silico by analysing molecular-dynamics (MD) data. Application of Onsager’s regression hypothesis to the macroscopic constitutive laws for multicomponent Stefan–Maxwell mass transport allows application of Casimir’s fluctuation theory, which shows how the decay rates of certain autocorrelation functions relate to macroscopic mutual diffusivities. The autocorrelation functions are then determined numerically, using periodic MD simulations in the Gibbs ensemble. As an illustrative example, we compute the diffusivities for a ternary Lennard–Jones gas mixture, a case where the diffusivities measured by MD can be compared to diffusivities determined analytically by the Chapman–Enskog kinetic theory. The ultimate application of this method will be to quantify component activities and transport properties in liquids, with the ultimate aim of studying electrochemical transport properties in electrolytic solutions.