Abstract
The electronic structure of the corundum-type transition-metal oxides $\chem{V_2O_3}$ and $\chem{Ti_2O_3}$ is studied by means of the augmented spherical wave method, based on density-functional theory and the local density approximation. Comparing the results for the vanadate and the titanate allows us to understand the peculiar shape of the metal $3d$ $a_{1g}$ density of states, which is present in both compounds. The $a_{1g}$ states are subject to pronounced bonding-antibonding splitting due to metal-metal overlap along the c-axis of the corundum structure. However, the corresponding partial density of states is strongly asymmetric with considerably more weight on the high-energy branch. We argue that this asymmetry is due to an unexpected broadening of the bonding $a_{1g}$ states, which is caused by hybridization with the $e_g^{\pi}$ bands. In contrast, the antibonding $a_{1g}$ states display no such hybridization and form a sharp peak. Our results shed new light on the role of the $a_{1g}$ orbitals for the metal-insulator transitions of $\chem{V_2O_3}$. In particular, due to $a_{1g}$-$e_g^{\pi}$ hybridization, an interpretation in terms of molecular orbital singlet states on the metal-metal pairs along the c-axis is not an adequate description.