  The measurement of the lattice-parameter of silicon by x-ray interferometry
assumes the use of strain-free crystals. This might not be the case because
surface relaxation, reconstruction, and oxidation cause strains without the
application of any external force. In a previous work, this intrinsic strain
was estimated by a finite element analysis, where the surface stress was
modeled by an elastic membrane having a 1 N/m tensile strength. The present
paper quantities the surface stress by a density functional theory calculation.
We found a value exceeding the nominal value used, which potentially affects
the measurement accuracy.
