  Vanadium dioxide, VO2, and its metal-insulator transition at T=340K continues
to receive considerable interest. The question whether the physics of the
insulating low-temperature phase is dominated by the Mott or the Peierls
scenario, i.e. by correlation or band effects, is still under debate. A recent
cluster dynamical mean field theory calculation [Biermann et al, Phys. Rev.
Lett., 94, 026404 (2005)] suggests a combination of both effects,
characterizing the transition as of a correlation assisted Peierls type. In
this paper we present a detailed analysis of the excitation spectrum of the
insulating M1 phase of VO2, based on this calculation. We implement a scheme to
analytically continue self-energies from Matsubara to real frequencies, and
study the physics of the strong interactions, as well as the corresponding
changes with respect to the density functional theory (LDA) band structure. We
find that in the M1 phase life-time effects are rather negligible, indeed
allowing for an effective band structure description. A frequency independent
but orbital dependent potential, constructed as an approximation to the full
cluster dynamical mean field self-energy, turns out to satisfactory reproduce
the fully interacting one-particle spectrum, acting as a scissors operator
which pushes the a1g bonding and egp bands apart and, thus, opening the gap.
