Density functional theory (DFT) were done with "Vienna ab initio simulations (VASP)" package . The Electron-ion interactions were described using all-electron projector augmented wave pseudopotentials, and "Perdew-Bruke-Ernzehof (PBE)" "generalized gradient approximation (GGA)" was used to approximate the electronic exchange and correlations . The plane-wave kinetic energy cut off of 520 eV was used. All the structures were optimized using a conjugate gradient scheme until the energies, and the components of forces reached 10-5 eV and 0.01 eV