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

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4 Traditional Molecular Dynamics Traditional MD numerically integrates Newton’s equation of motion over 3N 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.

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

6 Variational Gaussian Method {x i,  i } true free energy VG free energy

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

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9 DMD thermodynamics VG view DMD view

10 DMD kinetics nearest-neighbor network

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12 DMD is atomistic realization of regular solution model, with gradient thermo, long-range elastic interaction, and short-range 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.