  We define Wannier functions for interacting systems, and show that the
results on the localization of the Wannier functions for non-interacting
systems carry over to the Wannier functions for interacting systems. In
addition we demonstrate that the characterization of metals and insulators by
the decay properties of their respective density matrices does not only apply
to non-interacting, but also to interacting systems. As a prototypical example
of a correlated system we investigate the one-dimensional Hubbard model. We
propose an expression for the density matrix of that model, and derive a
relation between the decay constant of the density matrix and the gap.
