  We study Density Functional Theory models for systems which are
translationally invariant in some directions, such as a homogeneous 2-d slab in
the 3-d space. We show how the different terms of the energy are modified and
we derive reduced equations in the remaining directions. In the Thomas-Fermi
model, we prove that there is perfect screening, and provide decay estimates
for the electronic density away from the slab. In Kohn-Sham models, we prove
that the Pauli principle is replaced by a penalization term in the energy. In
the reduced Hartree-Fock model in particular, we prove that the resulting model
is well-posed, and provide some properties for the minimizer.
