Despite intense work over four decades the metal-insulator transition of VO₂ is still far from being understood. To some part this is due to the simultaneous occurance of a structural transformation, which fact hindered to clearly identify the structural or the electronic degrees of freedom as the origin of the transition. Several models have been proposed ranging from Peierls-, spin-Peierls- to Mott-Hubbard-type scenarios, which stress, to a different degree, the role of lattice instabilities, electron-phonon interaction and electronic correlations. Yet, neither of these pictures has so far been successful in explaining a broader range of the physical phenomena showing up in VO₂.

We present results of electronic structure calculations for VO₂ as based on density functional theory. As a calculational scheme we used the augmented spherical wave (ASW) method. Both the high-temperature rutile phase and the low-temperature monoclinic M₁ phase were considered. They differ mainly by two fundamental displacements occuring in the characteristic vanadium chains, namely, a V-V dimerization and a zigzag-like antiferroelectric V shift. For the rutile phase we find V 3d t_2g states near the Fermi energy, which have small admixtures from O 2p states due to π-type bonding. These vanadium bands fall into two groups: While the almost perfect one-dimensional dispersion of the V 3d_x²-y² states reflects the metal-metal overlap along the chains the remaining t_2g bands display a more three-dimensional dispersion. On going from the rutile to the monoclinic stucture, the V 3d_x²-y² states show a strong bonding-antibonding splitting due to the dimerization. In contrast, the other two bands shift to higher energies as a consequence of the antiferroelectric mode. The result is an effective energetic separation of both groups, which, however, still show a tiny semimetallic-like band overlap due to the shortcomings of the local density approximation.

We performed additional calculations for the insulating monoclinic M₂ phase of VO₂, where only half of the chains dimerize. The antiferroelectric shifts are limited to the remaining chains, which, in addition, display antiferromagnetic order. While we obtain for the dimerizing chains a similar picture as for the M₁ phase, lifting the spin degeneracy leads to splitting of the V 3d_x²-y² states on the zigzag chains. As a consequence, these states are again separated from the other t_2g bands with the tiny band overlap still remaining. To conclude, though the helping hand of electronic correlations seems to be needed for a complete gap opening, the metal-insulator transition of VO₂ arises predominantly from structural and magnetic instabilities of the rutile phase.