Diffusive Molecular Dynamics Ju Li Bill Cox Tom

Diffusive Molecular Dynamics Ju Li, Bill Cox, Tom Lenosky, Ning Ma, Yunzhi Wang Ohio State University

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Traditional Molecular Dynamics • Traditional MD numerically integrates Newton’s equation of motion over 3 N degrees of freedom, the atomic positions: • It is difficult to reach diffusive time scales using traditional MD due to timestep (~ ps / 100), which needs to resolve atomic vibrations. 4

Diffusive MD: The Idea Variational Gaussian Method Lesar, Najafabadi and Srolovitz, Phys. Rev. Lett. 63 (1989) 624. DMD ci: occupation probability (vacancy, solutes) Define mi for each atom, to drive diffusion Ferris wheel seen with long camera exposure time 5

Variational Gaussian Method VG free energy true free energy {xi, ai} 6

Comparison with Monte Carlo Lesar, Najafabadi and Srolovitz, Phys. Rev. Lett. 63 (1989) 624. 7

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DMD thermodynamics VG view DMD view 9

DMD kinetics nearest-neighbor network 10

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• DMD is atomistic realization of regular solution model, with gradient thermo, long-range elastic interaction, and shortrange coordination interactions all included. • DMD kinetics is “solving Cahn-Hilliard equation on a moving atom grid”, with atomic spatial resolution, but at diffusive timescales. • The “quasi-continuum” version of DMD can be coupled to well-established diffusion-microelasticity equation solvers such as phase-field method. • No need to pre-build event catalog. Could be competitive against kinetic Monte Carlo. 12