Discontinuity of the chemical potential in RDMFT for open-shell systems
  We employ reduced density-matrix functional theory in the calculation of the fundamental gap of open-shell systems. The formula for the calculation of the fundamental gap is derived with special attention to the spin of the neutral and the charged systems. We discuss the effects of different functionals as well as the changes due to different basis sets. Also, we investigate the importance of varying the natural orbitals for the calculation of the fundamental gap 
N. Helbig, N.N. Lathiotakis, E.K.U. Gross