  The measurement of the Avogadro constant by counting Si atoms is based on the
assumption that Si balls of about 94 mm diameter have a perfect crystal
structure up to the outermost atom layers. This not the case because of the
surface relaxation and reconstruction, the possible presence of an amorphous
layer, and the oxidation process due to the interaction with the ambient. This
paper gives the results of density functional calculations of the strain
components orthogonal to crystal surface in a number of configurations likely
found in real samples.
