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For slab model, we can regard the model as the sliced bulk material, in which the bonds between atoms are cut, forming termination layers. If we are considering the surface states or superficial properties of real materials, the unstaturated bonds at the surfaces act inevitable roles. In DFT atomistic model, unless the model is large enough, the bottom atomic layer usually interferes the charge distribution and charge transfer of the upper-most atomic layer - the surface layer we are interested in, by the finite number of atomic layers. To avoid this influence, a method called Pseudo-Hydrogen is well-adopted, to saturate the additional bonds located at the bottom atomic layer. By doing so, we can prevent the artificial and unphysical charge transfer from the bottom layer to the upper-most layer, allowing the prediction of real surface states distribution at the surfaces.

The implementation of the pseudo-hydrogen method requires basic understading of the bonding types of the target slab material in bulk state. In particular, the covalent bond in ZnO, where Zn2+ holds 6 outter-most electrons and O2- holds 2 outter-most electrons. The covalent bond formed in bulk ZnO crystal reflects a 3:1 ratio of contribution of a fully saturated system. A simple calculation can be done like

To preserve the surface unsaturated at the surface interested, the otherside of the system need to be saturated by the psuedo-Hydrogen atoms.