Periodic DFT calculations, including the electron density calculation, were carried out within the plane-wave pseudopotential formalism, using the Vienna ab initio simulation package (VASP) code version 5.4.4 . Projector augmented-wave (PAW) method was applied to describe the electron-ion interactions . Generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional was adopted to treat electron interaction energy . Grimme's semi-empirical DFT-D3 scheme with Becke-Johnson damping functions was used here to give a better description of interactions . A kinetic-energy cut-off of 600 eV was used to define the plane-wave basis set. The electronic Brillouin zone was integrated with the smallest allowed spacing between k -points (KSPACING) being 0.4 Å-1 , and the generated grid was centered at the Γ-point. The convergence threshold for self-consistency was set to 10 -6 eV during total energy and force calculations.