Shape, polarizability, and metallicity in silicon clusters

We compute the dipole polarizability for Si_n clusters across the prolate to compact shape transition region, n=20–28, and also for a prolate and compact isomer at n=50. We find a clear shape dependence in the calculated values, with prolate structures having systematically larger polarizabilities, and very different trends in per atom polarizabilities with cluster size. The shape dependence is not due to highest unoccupied molecular orbital (HOMO)-lowest unoccupied molecular orbital gap differences or to differences in the binding of the HOMO electron. Instead, charge density analyses show a metalliclike response of the clusters to an external field. In addition, the size trends for the calculated polarizabilities for the compact and prolate clusters are reproduced by the predictions of jellium models for spheres and cylinders, respectively, further suggesting that these small clusters exhibit metallic character.