The magnesium hydride stability and bonding have been studied using density functional theory (DFT) To this aim calculations on the electronic structure were performed We also modeled the bulk hydride with a Nb atom as a substitutional impurity Furthermore both systems were modeled containing different types of vacancies (Mg H or H-Mg complex) The crystal orbital overlap population for both the metal-metal and metal-hydrogen bonds was also computed The influence of vacancy like defects was studied through the calculation of the positron lifetimes in defected MgH2 and defected MgH2-Nb For the pure hydride, the results show an increment in the atom bonds in correlation with an increase of the positron localization reflected in a rise of the positron lifetimes On the other hand in all considered cases for Mg or/and H vacancies the presence of Nb reduces the hydride bond about 36% This decrease in the hydride stability was associated with a decrease in the probability of the positron localization and a consequently reduction of the positron lifetimes (C) 2010 Professor T Nejat Veziroglu Published by Elsevier Ltd All rights reserved