where G = E + pV -T S with the internal energy E, pressure p, volume V , temperature T , and entropy S. N i is the number of atoms in the system, and µ i is the chemical potential of species i, respectively. The Gibbs free energy is normalized to the surface areas of the two surfaces of the symmetric slab model. The contribution of the term pV can be neglected as shown in literature. Hence, Eq. 1 changes to