  The frequency-dependent exchange-correlation potential, which appears in the
usual Kohn-Sham formulation of a time-dependent linear response problem, is a
strongly nonlocal functional of the density, so that a consistent local density
approximation generally does not exist. This problem can be avoided by choosing
the current-density as the basic variable in a generalized Kohn-Sham theory.
This theory admits a local approximation which, for fixed frequency, is exact
in the limit of slowly varying densities and perturbing potentials.
