Ab initio calculations were used to study dehydrogenation energy in a set of systems, pure, Nb- or Zr-doped, free of defects and containing vacancy-like defects MgH2 (001) surface. The preferential location site for dopants was determined by means of occupation energy analysis. Zr atom prefers substitutional and interstitial sites while Nb locates on interstitial site close to the surface. The effect of vacancies in the geometric structure and in dehydrogenation energy was considered. It was found that MgH2 containing a Mg vacancy doped with Zr, in an interstitial site and the same system containing Mg H complex vacancy, modify the surface geometry and weakens the Mg H bonds making easier the dehydrogenation process. When the surface is doped with Nb despite any vacancy generation, the dehydrogenation is less probable energetically in comparison with Zr-doped. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.