  In this work we employ the $GW$ approximation in the framework of the
SternheimerGW method to investigate the effects of many-body corrections to the
band structures and Fermi surfaces of bulk and monolayer NbS$_2$. For the bulk
system, we find that the inclusion of these many-body effects leads to
important changes in the band structure, especially in the low-energy regime
around the Fermi level, and that our calculations are in good agreement with
recent ARPES measurements. In the case of a free-standing monolayer NbS$_2$, we
observe a strong increase of the screened Coulomb interaction and the
quasiparticle corrections as compared to bulk. In this case we also perform
calculations to include the effect of screening by a substrate. We report in
detail the results of our convergence tests and computational parameters, to
serve as a solid basis for future studies.
