ensemble (NEMD shock methods)

In a typical NEMD shock simulation, a shock wave is generated by a moving wall. The wall can be simulated in multiple ways:

  • wall_piston: A fixed layer of atoms.

  • wall_mirror: A momentum mirror that reflects atoms.

  • wall_harmonic: A harmonic potential that pushes atoms away. It is softer than a momentum mirror.

Any of these methods can generate a shock wave. While there are other possible methods, the above methods are usually sufficient.

Please note that the shock wave propagates in the x-direction. Another wall is placed on the opposite side of the cell to prevent atoms from escaping. It is important NOT to use non-periodic boundary conditions in the x-direction, as GPUMD currently does not handle it well. A vacuum layer is automatically added on the other side of the cell.

It is recommended to use this with the dump_shock_nemd keyword to get the spatial distribution of thermo information.

Syntax

The parameters are specified as follows:

ensemble wall_piston vp <vp> thickness <thickness>

  • <vp>: Indicates the velocity of the moving piston in km/s.

  • <thickness>: Defines the thickness of the wall in Angstroms. This keyword is optional with the default value of 20.

ensemble wall_mirror vp <vp>

  • <vp>: Indicates the velocity of the moving piston in km/s.

ensemble wall_harmonic vp <vp> k <k>

  • <vp>: Indicates the velocity of the moving piston in km/s.

  • <k>: Defines the strength of the harmonic wall in eV/A^2. This keyword is optional with the default value of 10.