Using density functional calculations, we studied substoichiometric Ge oxides through a series of crystalline models representing the various oxidation states. We evaluated deviations with respect to a bond-energy description in terms of penalty energies for intermediate oxidation states of Ge. Corresponding bond length variations and band gaps are also given. The calculated penalty energies are significantly lower than for Si indicating higher stability of the substoichiometric phase at Ge/GeO2 interfaces compared to Si/SiO2 interfaces. (C) 2009 Elsevier B.V. All rights reserved.