  The microscopic structure of the high-index Si$(331)-(12\times1)$ surface is
investigated combining scanning tunneling microscopy with ab initio
calculations. We present a structural model of the Si(331) surface, employing a
reconstruction element composed of six pentagons integrated to the structure of
the adjacent pentamer with an interstitial atom. We demonstrate that
appropriately arranged additional pentagons significantly lower the surface
energy of the high-index surface. The model predicts the existence of multiple
Si(331) buckled configurations with similar energies.
