DFT calculations were used to determine the activation energies (E a values) for the diffusion of defects such as O atoms (O i0 red sphere in the picture) in silicon (blue spheres). The migration paths were obtained from the nudged elastic band method. The activation energies calculated with four exchange-correlation functionals (E xc) were compared to experimental data. The E a values of "atomic-like" interstitials are mostly independent of E xc, but those of strongly bound impurities are sensitive to the choice of E xc.