  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.
