Our first-principles calculations are performed in the framework of spin-polarized density functional theory using projector augmented wave (PAW) potentials to describe the core electrons and the generalized gradient approximation (GGA) of Perdew, Burke, and Ernzernhof (PBE) for exchange and correlation as implemented in the Vienna Ab initio Simulation Package (VASP) based on a plane-wave basis set. A cutoff energy of 500 eV for the plane-wave expansion is used in all calculations and all structures are fully relaxed until the Hellmann-Feynman forces on all the atoms are less than 0.01 eV/Å. The lattice parameters and internal coordinates of the structures are fully relaxed to achieve the lowest energy configurations using conjugate gradient algorithm. Geometry optimization is carried out employing the conjugated gradient technique and the convergence for the total energy is set as 10 -6 eV.