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MD

MD: Molecular Dynamics
Definition:In MD, interactions between molecules or atoms are calculated for a small time interval.
Explanation:Between each particle in a MD simulation, a potential is applied and forces are derived from the potential gradient. Position and velocity for each atom or molecule are updated according to these forces in small discrete time intervals (~1 fs).

Depending on the type of the applied potential, it is possible to simulate systems in the order of ~100 to ~1000000 atoms for the time scales up to several nanoseconds. In this range the possible system, size largely depends on the complexity of the applied potential. Most of the time semi-empirical potentials are used for computational efficiency, though it is also possible to use ab initio methods. There exist a wide range of semi-empirical potentials where the compromise between accuracy and calculation time can be adjusted to meet the need. These potentials are fitted to approximate physical properties measured in experiments or derived from ab initio calculations for the respective element to be described. For metals, two huge classes of MD potentials are the embedded atom method (→EAM) and the modified EAM (→MEAM).

Inherently, a MD simulation only returns the position of every atom or molecule for each simulated time step. To derive the intended value of interest, post processing or visualization of ththe large amount of generated data is usually necessary.

There exist several MD implementations, which contain the same set of basic functionality, but have specialized in different directions. In the SFB, all groups performing MD simulations use the MD code LAMMPS.
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Diagram:

Image of a MD simulation box containing a bicrystal for a grain boundary mobility simulation. The atoms are colored according to their grain orientation. Periodic boundary conditions are applied in Y- and Z- direction, while the two crystals each have a free surface in X- direction.
SFB-Link:MD simulations are used in part projects A1, A6 and A10. In A1, MD is used to determine diffusion properties, in A6, to determine grain boundary mobility, and in A10, to create a catalogue of possible dislocation interactions.
References:D. C. Rapaport - The Art of Molecular Dynamics Simulation