This chapter discusses DFTB+ calculations where electrons are allowed to move (evolve in time). This allows the calculation, for example, of electronic absorption spectra if the electrons are initially perturbed from the ground-state by a Dirac delta pulse. More general external time dependent fields can also be applied, including arbitrary elliptical polarisation and pulses with various envelope functions.
Ehrenfest dynamics can also be performed, where both the electrons and ions move. In this methods nuclei are driven by the instantaneous expectation value of the force at each time for the moving electron density.
In this chapter we will provide example calculations of electronic spectra, driving of the electronic dynamics using external pulsed and continuous fields and driving of nuclear motion via electronic excitation.