  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.
