Effect of Size and Geometry on the Electronic Properties of Small Hydrogenated Silicon Clusters

We have calculated the geometrical characteristics and the electronic properties of small Si$_n$H$_m$ clusters, using the local-density approximation of the density functional theory and the pseudopotential approach. Both for passivated crystal fragments and for octasilacubane our results indicate nontrivial and so far unexpected effects of the cluster structure on the nature of the low-lying electronic excitations. Comparison with previous calculations as well as with experimental data, where available, is made. For the sake of comparison, calculations on (carbon) cubane are also presented.