The use of different level of complexity in theoretical chemistry allows to discuss the validity of the models of the reconstruction of the Si(100) surface and to create models of suboxides. Our ab-initio periodic Hartree-Fock study is in favour of a symmetric reconstrustion associated with an anti-ferromagnetic 2x2 arrangement of the spins of the dangling bonds. The assymetric reconstruction is isoenergetic. The use of a large cluster model treated at the semi-empirical levels AM1 & PM3 allows us to render the constraints of the surface. The important relaxation founded in this study is the combination of a vertical translation and a rotation. The network, the valency of the species can modify the kinetic of the relaxation. The first oxidation site is the dimer bond. the silicon atoms are then oxidated one by one.
