It was proposed that so-called magic number Si clusters of intermediate size correspond to structures in which exactly all surface atoms participate in reconstructions similar to bulk-Si surfaces. Models were given for the experimentally observed stable clusters of 33 and 45 atoms, which display features similar to the 7 x 7 and 2 x 1 reconstructions of the Si(111) surface. The nature of bonding in the energy-optimized atomic geometries was revealed, through first-principles electronic structure calculations, to be covalent. Effect of Surface Reconstruction on Stability and Reactivity of Si Clusters. E.Kaxiras: Physical Review Letters, 1990, 64, 551-4