The band gap discontinuity in DFT??? Hi everybody.... I've read about the band gap problem of Density functional theory, there is a discontinuity of the band gap when an electron add to the Kohn-Sham system I have two questions could anybody answer me? please 1- why this discontinuity happens anyway? 2- why we need to add an electron to the Kohn-Sham system to calculate the band gap? with my regards Nawzad A.