  We study the optical response of VO2 in the M1 insulating phase using methods
based on density functional theory in its most recent developments. We start
from a hybrid functional approach which may be a good starting point to carry
out many-body perturbation theory since, as we show, it gives a qualitatively
and to some degree quantitatively correct description of the insulating phase.
In order to calculate the dielectric function, first, using hybrid density
functional wavefunctions we have added GW corrections on top of the hybrid
density functional calculations and we solved the so-called Bethe Salpeter
equation to include effects of correlations of the electron-hole pair created
upon photon absorption. We find that the effects of electron correlations are
very important and they show up as a strong contribution of the electron-hole
interaction in calculating the effects of the Bethe-Salpeter equation in the
electron/hole pair excitation on top of the interacting group state.
