Another point worth mentioning is that the surface H coverage has a significant influence on the CLS, which has been less discussed in previous work. The influence of H adatoms on band structure can be divided into two parts. For one thing, the coupling between H1s and Osp3 hybrid orbitals gives rise to bonding and antibonding-like (localized) states at ca. -21 and -9 eV below Fermi level, respectively. For another, H1s also contributes to the extended states in conduction band, leading to the metallicity of ZnO (100) surface (Figure ). The results in Figure show that adsorbed H not only affects the surface oxygen CLS, but also the computed CLS value for oxygen in subsurface layers. The change of CLS with surface H density is not monotonic. With increasing H coverage from 0 to 1 ML, the CLS first increases and the decreases, and the ½ ML H coverage produces the largest shifts, where the maximum CLS for surface -OH, O and 2 nd layer O is 2.5, 0.06, and 0.91 eV, respectively. The O1s core level spectrum is simulated and the results are shown in Figure . The CLS from Figure is expanded using the Gaussian/Lorentzian product function with a mix ratio of 70/30 (GL (30)) and then added up to form the spectrum with a linewidth of 1.5eV, which is a typical linewidth under experimental conditions. The linewidth has a crucial effect on fitting results, which will be further discussed in the analysis of the Vo CLS. For simplicity, the weight of CLS of all O atoms is set to be the same, except for those in the surface bilayer, which is multiplied by a factor of 2 (see Figure for a detailed explanation). We have tested that the choice of CLS weight has little influence on the result of Δ B.E. Figure shows the simulated O1s core level spectra and fitting results. Figure displays the spectrum of the bare ZnO surface, where only one peak is present. The max. CLS is only +0.40/-0.49eV, which is too small to be distinguished, given the linewidth used in peak fitting. Figure -3d display the spectra for surfaces with different H coverage. There arises a shoulder peak related to the surface -OH group, whose Δ B.E. lies between 2.0-2.2 eV, depending on the H coverage.