First-principles study of crystalline and amorphous Ge(2)Sb(2)Te(5) and the effects of stoichiometric defects. 
Based on ab initio molecular dynamics simulations, we investigated the structural, electronic and vibrational properties of cubic and amorphous Ge(2)Sb(2)Te(5) (GST) phase change material, focusing in particular on the effects of defects in stoichiometry on the electronic properties.  It turned out Ge/Sb deficiencies (excess) in the cubic phase induce a shift of the Fermi level inside the valence (conduction) bands.  In contrast, the amorphous network is flexible enough to accommodate defects in stoichiometry, keeping the Fermi level pinned at the center of the bandgap (at zero temperature).  Changes in the structural and electronic properties induced by the use of hybrid functionals (HSE03, PBE0) instead of gradient corrected functionals (PBE) are addressed as well.  Analysis of vibrational spectra and Debye-Waller factors of cubic and amorphous GST is also presented.