where A is the area of the surface and the factor 2 reflects the two equivalent surfaces that are created. Compared to calculating the surface energy directly from Equation where the bulk DFT energy must be explicitly stated, For the (221) Miller index, the 50 Å slab contained 878 atoms which makes the relaxation computationally too expensive to justify even at the GGA level, so only three data points are used in the FM fit. For completeness, the cleavage energies of unrelaxed slabs were also calculated using the same method. Cleavage and surface energies of all slabs can be found in Table .