*Ab initio* total-energy density functional theory is used to investigate the low temperature (LT) monoclinic form of Mg_{2}NiH_{4}. The calculated minimum energy geometry of LT Mg_{2}NiH_{4} is close to that determined from neutron diffraction data, and the NiH_{4} complex is close to a regular tetrahedron. The enthalpies of the phase change to high temperature (HT) pseudo-cubic Mg_{2}NiH_{4} and of hydrogen absorption by Mg_{2}Ni are calculated and compared with experimental values. LT Mg_{2}NiH_{4} is found to be a semiconductor with an indirect band gap of 1.4 eV. The optical dielectric function of LT Mg_{2}NiH_{4} differs somewhat from that of the HT phase. A calculated thin film transmittance spectrum is consistent with an experimental spectrum.

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%2 LBNL-47519 %& 4879 %R 10.1063/1.1454206