The (100) face of beta-cristobalite covered by geminal hydroxyls has been taken as a model for the silica surface in order to describe the hydrogen bonding and the condensation of the surface hydroxyls of silica Periodic Hartree-Fock calculations have been performed for slabs of five atomic layers. The optimized orientation of the hydroxyls corresponds indeed to hydrogen bonds at the surface. As a result, the surface atoms are slightly more negative and the hydroxyls have a specific orientation : one pointing outside the surface plane and the other one directed toward the bulk. The H-bond energy is evaluated to 25 kJ.mol(-1). The dehydration, leading to siloxane bridges, is endothermic by approximate to 200 KJ.mol(-1). It must be accompanied by a surface restructuring implicating more than only the first silicon and oxygen layers.