This variable defines the factor between the timescale of ionic
and electronic movement. It allows reasonably fast
Born-Oppenheimer molecular-dynamics simulations based on
Ehrenfest dynamics. The value of this variable is equivalent to
the role of $\mu$ in Car-Parrinello. Increasing it
linearly accelerates the time step of the ion
dynamics, but also increases the deviation of the system from the
Born-Oppenheimer surface. The default is 1, which means that both
timescales are the same. Note that a value different than 1
implies that the electrons will not follow physical behaviour.
According to our tests, values around 10 are reasonable, but it
will depend on your system, mainly on the width of the gap.
Important: The electronic time step will be the value of
TDTimeStep divided by this variable, so if you have determined an
optimal electronic time step (that we can call dte), it is
recommended that you define your time step as:
TDTimeStep = dte * TDIonicTimeScale
so you will always use the optimal electronic time step
(more details).