Size effects in surface reconstructed $<100>$ and $< 110>$ silicon nanowires

The geometrical and electronic structure properties of $<100>$ and $<110>$ silicon nanowires in the absence of surface passivation are studied by means of density-functional calculations. As we have shown in a recent publication [R. Rurali and N. Lorente, Phys. Rev. Lett. {bf 94}, 026805 (2005)] the reconstruction of facets can give rise to surface metallic states. In this work, we analyze the dependence of geometric and electronic structure features on the size of the wire and on the growth direction.