All the calculations were conducted by density functional theory (DFT) method using the periodic plane wave pseudopotential package CASTEP, with exchange-correlation potential approximated by the Perdew, Burke and Ernzerhof functional which is a generalized gradient approximation (GGA). Ultrasoft pseudopotentials (USPPs) generated on-the-fly (OTFG) were used for geometry optimization and ground state calculation. Excited core OTFG USPPs were employed for CLS calculation. The lattice parameters of ZnO were optimized to a=3.284 Å and c=5.261 Å, which were then adopted both for building slab and supercell models. Careful convergence tests were performed against plane wave cutoff, k-point grid, supercell size, slab size, and vacuum thickness. The convergence of total energy for each model was 0.001eV/atom. The convergence of CLS for surface and supercell model were smaller than 0.02eV and 0.07eV, respectively, which are better than experimental XPS accuracy.