(Experimental) This variable allows you to specify a
time-dependent function describing the displacement of the ions
from their equilibrium position: $r(t) = r_0 + \Delta
r(t)$. Specify the displacements dx(t), dy(t), dz(t) as
follows, for some or all of the atoms:
The displacement functions are time-dependent functions and should match one
of the function names given in the first column of the TDFunctions block.
If this block is set, the ions will not be affected by any forces.