Many-body effects in TiSe2: Can GW describe an Excitonic Insulator?
  We present both theoretical ab initio GW and experimental angle-resolved photoemission (ARPES) and scanning tunneling (STS) spectroscopy results on TiSe2. With respect to the density-functional Kohn-Sham metallic picture, the many-body GW self-energy leads to a ~ 0.2 eV band gap insulator consistent with our STS spectra at 5 K. The band shape is strongly renormalized, with the top-of-valence moved towards a circle of points away from \Gamma, arising in a mexican hat feature typical of an excitonic insulator. Our calculations are in good agreement with experiment. 
M. Cazzaniga, H. Cercellier, M. Holzmann, C. Monney, P. Aebi, G.   Onida, and V. Olevano
cond-mat.str-el cond-mat.other