Ab initio theory of the Si(111)-(7×7) surface reconstruction: A challenge for massively parallel computation

An ab initio investigation of the Si(111)-(7×7) surface reconstruction is undertaken using the state of the art in massively parallel computation. Calculations of the total energy of an ∼700 effective-atom supercell are performed to determine (1) the fully relaxed atomic geometry, (2) the scanning-tunneling-microscope images as a function of bias voltage, and (3) the energy difference between the (7×7) and the (2×1) reconstructions. The (7×7) reconstruction is found to be energetically favorable to the (2×1) surface by 60 meV per (1×1) unit cell.