Density Functional Theory (DFT) calculations have been used for calculating the elastic and thermodynamic properties of Mg, Nb, and their hydrides. The study is motivated by a previous work that showed constraints arising from Mg/Nb layer interfaces de-stabilized the magnesium hydride and resulted in decomposition temperatures much lower than in the bulk state. The calculated mechanical properties and the difference in the volume expansion of Mg- and Nb-hydrides support the ideas of large, de-stabilizing mechanical stresses arising from significant volume expansion incompatibilities in Mg/Nb multilayers. The stress and corresponding formation of metastable phases result in significant destabilization of MgH2 and provide ultra-low hydrogen desorption temperature in Mg/Nb multilayers. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.