Energetics in the initial stage of oxidation of silicon

We present microscopic calculations of the energetics in the initial stage of oxidation of Si. We find that an ${\mathrm{O}}_2$ molecule penetrates the oxygen-covered surface, dissociates near a bond-center site, and forms a peculiar bond configuration with Si atoms exothermically. We also find that the bond formation results in several oxidation states of the Si atoms at the interface and in a swelling of the Si-O-Si bond network from the surface.