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
td/td.F90 : 206
call parse_variable(namespace, 'TDIonicTimeScale', M_ONE, td%mu)