  We report on ab-initio calculations of the two-dimensional systems MoS2 and
NbSe2, which recently were synthesized. We find that two-dimensional MoS$_2$ is
a semiconductor with a gap which is rather close to that of the three
dimensional analogue, and that NbSe$_2$ is a metal, which is similar to the
three dimensional analogue of this compound.
  We further computed the electronic structure of the two-dimensional hexagonal
(graphene like) lattices of Si and Ge, and compare them with the electronic
structure of graphene. It is found that the properties related to the Dirac
cone do not appear in the case of two-dimensional hexagonal germanium, which is
metallic, contrary to two-dimensional hexagonal silicon, which has an
electronic structure very similar to the one of graphene, making them possibly
equivalent.
