We present results of generalized gradient approximation $\left(\text{GGA}\right)+U$ calculations for the “$d^0$ magnet” ${\text{RbO}}_2$, where magnetic properties are due to partially filled oxygen $p$ orbitals. We show that on-site interactions on the oxygen sites lead to a strong tendency toward the formation of an orbitally polarized insulating state in contrast to the half-metallic behavior predicted for this class of compounds within pure local-spin-density approximation and/or GGA. The obtained energy differences between different orbitally ordered configurations are sizable, indicating an orbital ordering temperature higher than the antiferromagnetic Néel temperature of $\sim 15\text{K}$. Our results demonstrate the importance of correlation effects in $p$ electron magnets such as ${\text{RbO}}_2$, which can serve as an important benchmark system to study correlation effects in a “clean” system where unoccupied $p$ states are not defect induced.

• Received 24 September 2009

DOI:https://doi.org/10.1103/PhysRevB.80.140411