The role of intrinsic defects has been investigated to determine binding
energies and the electronic structure of Gd complexes in ZnO. We use
density-functional theory and the GW method to show that the presence of
vacancies and interstitials affect the electronic structure of Gd doped ZnO.
However, the strong localization of the Gd-f and d states suggest that
carrier mediated ferromagnetism in this material may be difficult to achieve