However, to separate the surface from bulk point defects requires a slab model much larger than the current one, which is very computationally costly yet with little added value. Thus, we take the view that it is more reasonable to model these points defect in a bulk supercell model. Figure and show the local geometry of the relaxed structures of a ZnO 4×4×3 supercell containing a single Vo 2+ and Vo 0 , respectively. The four adjacent Zn atoms around Vo 2+ have an outward relaxation of 23% (Figure ), while their counterparts in Vo 0 region show an inward displacement (Figure ), which is consistent with previous studies on the defect configuration of VO 0 in ZnO In Vo 0 , the dangling bonds of the four adjacent Zn atoms form a localized defect state in band gap, leading to the inward lattice relaxation; while for Vo 2+ , no such states are present in the band gap. Figure displays the charge distribution of the localized defect state of Vo 0 . Alvaro et al. reported a maximum CLS of only 0.2 eV for the Vo 2+ defect at the surface using a slab model, which is much smaller than our results. As we explained above, the chemical shifts caused by surface and point defects are not independent from each other, making it impossible to extract the real CLS of Vo 2+ in bulk under such conditions. Figure shows that CLS of Vo 0 is much smaller than that of Vo 2+ , largely attributed to its less distorted structure. The max. CLS of Vo 0 is merely 0.12eV.