Charge-dependent migration pathways for the Ga vacancy in GaAs
  Using SIEST-ART, a combination of the local-basis \textit{ab-initio} program SIESTA and the activation-relaxation technique (ART nouveau) we study the diffusion mechanisms of the gallium vacancy in GaAs. Vacancies are found to diffuse to the second neighbor using two different mechanisms, as well as to the first and fourth neighbors following various mechanisms. We find that the height of the energy barrier is sensitive to the Fermi-level and generally increases with the charge state. Migration pathways themselves can be strongly charge-dependent and may appear or disappear as a function of the charge state. These differences in transition state and migration barrier are explained by the charge transfer that takes place during the vacancy migration. 
Fedwa El-Mellouhi, Normand Mousseau