Reduced Density Matrix Functional for Many-Electron Systems
  Reduced density matrix functional theory for the case of solids is presented and a new exchange correlation functional based on a fractional power of the density matrix is introduced. We show that compared to other functionals, this produces more accurate results for both finite systems. Moreover, it captures the correct band gap behavior for conventional semiconductors as well as strongly correlated Mott insulators, where a gap is obtained in absence of any magnetic ordering. 
S. Sharma, J. K. Dewhurst, N. N. Lathiotakis, E. K. U. Gross