As desorption process from GaAS(001) surface under atmospheric pressure of hydrogen is investigated by ab initio clculation. The value of the activation energy obtained from the calculations is close to the experimental value by in situ gravimetric (GM) method during the (1×1) to (2×4) transition. Our results show that H-adsorbed (2×4) reconstructed surfaces exist under the H2 ambient, and then As atoms on the surface desorb from the surface as As_2H_2 molecules.
