The role of Coulomb interaction on the electronic properties of monolayer NiX$_2$ (X = S, Se): A DFT+U+V study

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.