The OH bond near the apex of Au 20 is prone to breaking firstly, attributed to the initial structural arrangement. As Figure shows, we set one of H atom of H 2 O to bind with Au atom in the apex of Au 20 and the other to be away from the apex. With this structural arrangement, the symmetry of H 2 O breaks down. As the MO of No. 125 in Figure (c) shows, the wavefunction overlap between two OH bonds of H 2 O and Au 20 will be different, indicating that their ability to accept electrons will be different. With this regard, the asymmetric dissociation appears in PDWS process.