  The origin of the gap in NiS2 as well as the pressure- and doping-induced
metal-insulator transition in the NiS2-xSex solid solutions are investigated
both theoretically using the first-principles band structures combined with the
dynamical mean-field approximation for the electronic correlations and
experimentally by means of infrared and x-ray absorption spectroscopies. The
bonding-antibonding splitting in the S-S (Se-Se) dimer is identified as the
main parameter controlling the size of the charge gap. The implications for the
metal-insulator transition driven by pressure and Se doping are discussed.
