Vibrational signatures for low-energy intermediate-sized Si clusters

We report low-energy locally stable structures for the clusters ${\mathrm{Si}}_{20}$ and ${\mathrm{Si}}_{21}$. The structures were obtained by performing geometry optimizations within the local density approximation. Our calculated binding energies for these clusters are larger than any previously reported for this size regime. To aid in the experimental identification of the structures, we have computed the full vibrational spectra of the clusters, along with the Raman and IR activities of the various modes using a recently developed first-principles technique. These represent, to our knowledge, the first calculations of Raman and IR spectra for Si clusters of this size. © 1996 The American Physical Society.