Density functional theory (DFT) with the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) was used as implemented in the VASP code 20 for relaxing structures and calculating forces on atoms. A plane-wave energy cutoff of 600 eV was adopted in these calculations. The Brillouin-zone integrations were performed using a Γ-centered regular 24 × 24 × 1 k-point mesh within the Monkhorst-Pack scheme. The convergence criteria for electronic and ionic relaxations were set to 10 -6 eV and 10 -2 eV/ Å, respectively.