  In principle, we should not need the time-dependent extension of
density-functional theory (TDDFT) for excitations, and in particular not for
Molecular Dynamics (MD) studies: the theorem by Hohenberg and Kohn teaches us
that for any observable that we wish to look at (including dynamical properties
or observables dependent on excited states) there is a corresponding functional
of the ground-state density. Yet the unavailability of such magic functionals
in many cases (the theorem is a non-constructive existence result) demands the
development and use of the alternative exact reformulation of quantum mechanics
provided by TDDFT. This theory defines a convenient route to electronic
excitations and to the dynamics of a many-electron system subject to an
arbitrary time-dependent perturbation. This is, in fact, the main purpose of
inscribing TDDFT in a MD framework -the inclusion of the effect of electronic
excited states in the dynamics. However, as we will show in this review, it may
not be the only use of TDDFT in this context. In this manuscript, we review two
recent proposals: In Section 1.2, we show how TDDFT can be used to design
efficient gsBOMD algorithms -even if the electronic excited states are in this
case not relevant. The work described in Section 1.3 addresses the problem of
mixed quantum-classical systems at thermal equilibrium.
