2. MOVING ATOMS: stopping power for H moving along a channel in Ge¶

In this example we will see how to simulate the dynamics of an energetic hydrogen atom moving through a channel in Germanium.

The input file hydrogen-in-Ge.0.fdf contains a cell with 8 Germanium atoms in the fcc phase, and an interstitial Hydrogen. It initializes the TD-DFT calculation by writing a hydrogen-in-Ge.TDWF and a hydrogen-in-Ge.TDXV files

Edit this latter file to define an initial velocity on H along the (001) direction. Remember that XV files contain the atomic positions and velocities in Bohrs and Bohrs/fs. For example, the last line should read:

2     1       0.000000000       1.320400000       0.000000000          0.000000000       0.000000000      20.000000000


where the H atom is set to move with 20Bohr/fs along the z axis.

Now we can start the dynamic evolution of the system. The file hydrogen-in-Ge.move.fdf defines the simulation parameters:

MD.TypeOfRun               TDED
MD.FinalTimeStep            500
TDED.TimeStep               1.0000000000E-03 fs
TDED.Nsteps                 10


We save the total energy of the system (and the eigenvalues) at each time-step, and the final wavefunction using these lines:

TDED.Write.Etot               T
TDED.WF.Save                  T


Finally, note that:

• We are moving ONLY the H atom, keeping frozen the atomic positions of Ge atoms in the lattice:

%block GeometryConstraints
position from  1 to 8
%endblock GeometryConstraints

• Due to the periodic boundary conditions, when the H atom leaves the simulation box, another periodic image automatically enters again in the cell. Larger supercells would be needed to avoid over-heating the system.

After the simulation, we can plot the total energy, which shows the lost of energy due to the electron heating induced by the fast moving H atom. The periodic oscillations are due to the host lattice periodicity. You can repeat the example using different simulation parameters for the time-propagation, but also for the initial conditions of the moving H atom.