P and B diffusion has been modeled in Ge using ab initio methods along with the formation energies and electrical levels of various P x V y defects expected to be important in the deactivation of P in heavily n-doped Ge. The calculated activation barrier for B diffusion is found to be substantially lower than the measured barrier. However, the exceptionally large pre-exponential factor in the measured diffusivity points to a Meyer–Neldel rule operating and accounting for the discrepancy. The magnitude of the theoretical diffusivity is about a factor 10 lower than observed. For P diffusion, the experimental and theoretical results are in much closer agreement. The formation energy calculations show that all P x V y clusters are stable with respect to their component defects, and all but P 4 V are predicted to introduce acceptor levels into the band gap. A simple analysis of possible formation mechanisms and Coulombic contributions suggests that as in Si, P 3 V is the most important compensating center in heavily n-doped Ge.