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A first-principles method has been applied to the investigation of oxygen vacancies in ultrathin SiO2 gate dielectric and their influence on the gate leakage current. From the energy point of view, the most favorable site for a single vacancy has been determined to be at the Si/SiO2 interface. The formation energies of two neutral vacancies show, in general, an attractive interaction between two defects. Our results also indicate a correlation between the leakage current and the position of vacancies. As the number of vacancies increases in the oxide layer, the leakage current rises almost exponentially. A chain of five vacancies results in an increase of the leakage current by more than three orders of magnitude, which could be considered as gate-dielectric breakdown.