Group-IV covalent clusters: ${\mathrm{Si}}_{45}$ and ${\mathrm{C}}_{44}$ versus ${\mathrm{Si}}_{44}$ and ${\mathrm{C}}_{45}$

Kaxiras has recently proposed a physically appealing structure for the relatively inert ${\mathrm{Si}}_{45}$ cluster observed by Elkind, Alford, Weiss, Laakanen, and Smalley. Using empirical potentials and all-electron local-density-functional calculations, we have found that removing the central atom from this structure forms a 44-atom fullerenelike silicon cluster with a nearly identical atomization energy per atom, but removing the central atom from the corresponding 45-atom carbon structure forms a 44-atom carbon fullerene with a significantly higher atomization energy. This key difference between these large silicon and carbon clusters is consistent with experimentally observed cluster distributions and is explained based on the greater tendency for π bonding and higher strain energy in carbon.