An atomistic model of substoichiometric germanium oxide is generated through ab initio molecular dynamics. The resulting structure shows a predominance of threefold coordinated Ge and O atoms. We also generate substoichiometric models through bond-switching Monte-Carlo simulations, which preserve the fourfold Ge and the twofold O coordinations. These differing structures are energetically competitive. Alignment of their electron densities of states to that of GeO2 reveals that the band-gap reduction is similar for both structures, mainly occurring through a shift of the valence band edge. (C) 2010 Elsevier B.V. All rights reserved.