  The electronic structure of the vanadium dioxide VO_2 in the tetragonal R and
two monocinic M_1 and M_2 structural modifications was calculated in frames of
the local-density functional (LDA) approach and the LSDA + U formalism of
correction for correlation effects. Based on the results of calculation, we
argue in favor of the Mott-Hubbard mechanism of the metal-insulator transition.
It is shown that the transition is accompanied by the change of the orbital in
which the 3d^1 electron of the V^{4+} ion is localized. The symmetry of the
filled orbital is determined by the local oxygen environment of the vanadium
atoms in each structural phase. Depending on the dispersion of the band that
corresponds to the orbital occupied, the investigated compound can be either a
metal or an insulator.
