  We examine various integration schemes for the time-dependent Kohn-Sham
equations. Contrary to the time-dependent Schr\"odinger's equation, this set of
equations is non-linear, due to the dependence of the Hamiltonian on the
electronic density. We discuss some of their exact properties, and in
particular their symplectic structure. Four different families of propagators
are considered, specifically the linear multistep, Runge-Kutta, exponential
Runge-Kutta, and the commutator-free Magnus schemes. These have been chosen
because they have been largely ignored in the past for time-dependent
electronic structure calculations. The performance is analyzed in terms of
cost-versus-accuracy. The clear winner, in terms of robustness, simplicity, and
efficiency is a simplified version of a fourth-order commutator-free Magnus
integrator. However, in some specific cases, other propagators, such as some
implicit versions of the multistep methods, may be useful.
