up causes growth of pyramids sizes (see Fig. \[figure3\]). By the {211} pyramid we mean the pyramid built of (211), (121), and (112) facets.

![Snapshot of the surface at T=0.72 on warming.[]{data-label="figure3"}](fig3){width="8cm"}

A big jump of the surface energy is observed at $T= 1.26$ where the phase transition to disordered phase occurs. On the other hand, discontinuous change of the surface energy is observed at $T= 1.20$ when the system is cooling down. The hysteresis loop seen in Fig. \[figure2\] indicates that the phase transition is of first order. The surface is undercooled or overheated because system rest in metastable state separated from the stable state by the free energy barrier. The hysteresis effects, dependence of temperature of the phase transition on cooling and warming, has been observed in LEED experiment [@song95] for Pd/Mo(111). When system is cooling down we observe formation of large pyramids with defected faces and edges just below the temperature of the phase transition (see Fig. \[figure4\]). Facets of similar shapes and sizes were observed at this temperature on warming. During further cooling defects disappear and pyramids take nearly ideal shapes at $T\approx 1$. Therefore the dependence of surface energy on cooling is very week in 