Depending on the nature of the upper and lower surfaces, the differences can be significant. To resolve this problem, we designed a modified system consisting of two of the original asymmetric slabs. These are arranged such that the resulting new slab is symmetric, as illustrated in Fig. . Note that different from previous calculations, the original slab comprises 6 layers rather than 5 layers, with two unrelaxed layers and 4 fully relaxed layers, to allow for a continuous ABCABC… type stacking.