nearly ten times smaller than in the disordered phase because in former case only $\sim 1/3$ surface atoms contribute to S, i.e., surface atoms placed on facets of the same orientation.

Discussion
==========

We have presented here the BCCSOS model to study overlayer induced faceting. Although it might seem that model is very simple it gives many results in agreement with experiments. It has been shown that bcc(111) surface covered by a physical monolayer undergoes reconstruction upon annealing. We obtained two types of reconstructed surfaces: faceted {211} covered by 3-sided pyramids as observed in Pd/Mo(111) [@mad96] and faceted {011} surface found in Pd/Ta(111) [@szuk02]. The sizes of facets depend on annealing temperature and annealing time. At high temperature the phase transition to disordered phase occurs. We have shown that LEED diffraction patterns can be calculated, in kinematic approximation, in warming up and cooling down processes for different electron energies. In dependence of diffracted intensities on temperatures the hysteresis effect was observed close to temperature of the phase transition.

On the other hand, we can investigate some properties not reported in experimental papers. First of all the phase transition can be studied in details using this model. It is shown that 