with The calculations shown in Fig. indicate that OH adsorption is favorable on the defective 2×2-2D-1H surface, but not at all on 2×2-2D-2H. With that, the observed OH groups in the UP spectra could further indicate apparent P dangling bonds at the prepared InP(001) surface. In case of the 2D-1H surface, the hydroxyl group adsorbs on the P dimer dangling bond, saturating the half-filled dangling P orbital resulting from H desorption. Accordingly, the mid-gap surface state is removed and the band structure resembles that of the surface without defects.