Structure of nanoscale silicon clusters

Using the Car-Parrinello method, we have obtained unforeseen structures for the low-lying isomers of ${\mathrm{Si}}_{45}$ and other midsized Si clusters. They are formed by two shells of atoms, the outer one (cage) being fullerenelike and inner one (core) consisting of a few atoms saturating dangling bonds. These novel structures provide for the first time a consistent interpretation of the available experimental data, including the reactivity trends and the structural transition at a size of ∼25 atoms.