In addition to Oi, we would like to point out another possible origin for the misinterpreted defect peak. As has been illustrated in Figure , apart from the surface -OH species, the O atoms right underneath the surface may also give rise to high CLS. For example, in the ½ ML H model, O atoms in the 2 nd layer have a max. CLS as large as 0.91eV. Since the CLS is sensitive to the surface H density, a more focused study on this may reveal certain H coverages that give rise to even larger CLS, which may lead to distinguishable features in the XPS spectrum, but this is beyond the scope of the present study. Figure and 10b shows the Mulliken charge analysis before and after excitation. According to potential model, CLS changes linearly with partial atomic charge, which proves correct for some molecules. Yet for large and complicated systems, partitioning of charges to atoms itself can be a big problem. It can be seen that in the same surface (e.g. 1/4 ML H) or defect model, CLS has a roughly positive relation with Mulliken charge. However, comparison between different systems makes little sense. Even in the same model, the relation between CLS and atomic charge is not strict.