  The electronic structure of Nickel dichalcogenides, NiS$_2$ and NiSe$_2$, in
monolayer form, is studied employing first-principles methods. We assess the
importance of band ordering, covalency and Coulomb interactions in the ground
state of these systems. Hybrid functional results are compared with standard
functionals and also with Hubbard-corrected functionals to systematically
address the role of electronic interactions and localization. We found that
mean-field correlation realized by intersite Hubbard interactions are directly
linked to the magnitude of the energy band gap, giving compelling evidence for
the presence of a charge transfer insulating phase in these materials.
