Correlations in Ferropnictides

The strength of correlations in the ferropnictide superconductors is still under debate. While arguments for an electron-electron interaction $U$ of $5$eV have been made, some experimental results support a $U$ of merely $1$eV. Density functional theory in the local spin density approximation (LSDA) seems to describe several aspects of the electronic structure quite reasonably, which would also support a smaller $U$. However, the unusually large error of the calculated lattice structure remains a puzzle. We discuss the influence of correlations on the electronic structure and the properties of the ferropnictides in the framework of LSDA+U calculations.