Absence of large compressive stress on Si(111)

The surface stress tensor is calculated from first principles for several Si (111) 11, 21, and 22 surfaces. Contrary to widely held expectations, only a weak compressive stress is found for the unreconstructed 11 surface. Reconstructions involving -bonded chains or adatoms are predicted to be under substantial tensile stress. The results indicate that relief of surface stress is not the driving force for the 77 reconstruction. The stress is shown to be useful in prediction of the effect of externally applied strain, which can drive changes in the surface reconstruction pattern.