A Coupled Molecular Dynamics/Kinetic Monte Carlo Approach for Protonation Dynamics in Extended Systems.

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We propose a multiscale simulation scheme that combines first-principles Molecular Dynamics (MD) and kinetic Monte Carlo (kMC) simulations to describe ion transport processes. On the one hand, the molecular dynamics trajectory provides an accurate atomistic structure and its temporal evolution, and on the other hand, the Monte Carlo part models the long-time motion of the acidic protons. Our hybrid approach defines a coupling scheme between the MD and kMC simulations that allows the kMC topology to adapt continuously to the propagating atomistic microstructure of the system. On the example of a fuel cell membrane material, we validate our model by comparing its results with those of the pure MD simulation. We show that the hybrid scheme with an evolving topology results in a better description of proton diffusion than a conventional approach with a static kMC transfer rate matrix. Furthermore, we show that our approach can incorporate additional dynamical features such as the coupling of the rotation of a side group in the molecular building blocks. In the present implementation, we focus on ion conduction, but it is straightforward to generalize our approach to other transport phenomena such as electronic conduction or spin diffusion.