where the slab consists of a few layers of the respective material (Fig. ). A denotes the slab surface area, and Eslab is the total energy of the slab. In eq. (1a), Ebulk is the total energy of a corresponding bulk unit cell. Alternatively, one can also use eq. (1b), where the bulk energy is given by the number (ni) and the bulk energies of the respective atoms. These equations assume that the two sides of the slab have similar surface energies. However, as mentioned before, this neglects the fact that the atoms on the bottom surface are commonly not relaxed. In the case of bimetallic surfaces (Fig. ), this furthermore neglects that the two surfaces differ in their chemical composition. Consequently, one again obtains an average value of the surface energy, with unknown contributions of the upper and lower surface.