  Time-dependent current-density-functional theory (TDCDFT) provides an in
principle exact scheme to calculate efficiently response functions for a very
broad range of applications. However, the lack of approximations valid for a
range of parameters met in experimental conditions has so far delayed its
extensive use in inhomogeneous systems. On the other side, in many-body
perturbation theory (MBPT) accurate approximations are available, but at a
price of a higher computational cost. In the present work the possibility of
combining the advantages of both approaches is exploited. In this way an exact
equation for the exchange-correlation kernel of TDCDFT is obtained, which opens
the way for a systematic improvement of the approximations adopted in practical
applications. Finally, an approximate kernel for an efficient calculation of
spectra of solids and molecular conductances is suggested and its validity
discussed.
