vector of incident electron, scattered electron, respectively. Position of surface atoms ${\bf R}_j$ is expressed by column height $h_{r_j}$ in site ${r_j}=(x_j, y_j)$ in the following way ${\bf R}_j= a(3 \sqrt{2} x_j, \sqrt{6} y_j, \sqrt{3} h_{r_j})/6$. In a SOS model not all atoms from column tops are surface atoms. Therefore in calculation of diffracted intensity sometimes the shadowing factor is used [@tosat96]. In this paper we consider the contribution to $S$ only from surface atoms. As a surface atom in any configuration we choose the atom from the top of column which has at least four lower nearest neighbors. In the case when number of higher columns is equal to 3 then atom is regarded as surface atom if is not surrounded by the higher neighbors. Such case occurs for atoms on {110} facets. We will calculate structure factor only for a set of ${\bf k}_f$, chosen in such a way to describe diffraction on faces (111) and {211}. For each of these faces we construct a pair of inverse lattice vectors ${\bf g}_1, {\bf g}_2$ parallel to the face, e.g., for (121) ${\bf g}_1=2\pi/a(\sqrt{2}/2, 0, 0)$ and ${\bf 
g}_2=2\pi/a(0, 4\sqrt{6}/9, 2\sqrt{3}/9)$. Then, the wave vectors of scattered electrons, ${\bf 