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 t2g 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 3dx2-y2 states reflects the metal-metal overlap along the chains the remaining t2g bands display a more three-dimensional dispersion. On going from the rutile to the monoclinic stucture, the V 3dx2-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 3dx2-y2 states on the zigzag chains. As a consequence, these states are again separated from the other t2g 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.