  The response of an extended periodic system to a homogeneous field (of
wave-vector $q=0$) cannot be obtained from a $q=0$ time-dependent density
functional theory (TDDFT) calculation, because the
  Runge-Gross theorem does not apply. Time-dependent {\em current}-density
functional theory is needed and demonstrates that one key ingredient missing
from TDDFT is the macroscopic current. In the low-frequency limit, in certain
cases, density polarization functional theory is recovered and a formally exact
expression for the polarization functional is given.
