The results are in good accordance with experimental data, with Δ B.E in the range 1.8 -2.1 eV. We do not include surface defects in this model, because the effect of surface and point defects on CLS would interfere if the slab model is not large enough, making it impossible to extract the real chemical shifts of point defects in bulk. In reality, given the surface to volume ratio in films, the surface only accounts for a small part of the detection depth of XPS, and the majority of signals comes from the bulk part. Therefore, it is the defects in bulk that decide the spectrum features.