The search for the ground state structures through stress-free geometry optimizations calculations onto the ground state was performed with calculations within the plane-wave Vienna Ab initio Simulation Package (VASP) code . Based on DFT, VASP uses projector augmented wave (PAW) method for the atomic potentials with all valence states, especially in regard of such light elements as B, C, and N. The exchange-correlation (XC) effects treated at equal level in DFT were considered using the generalized gradient approximation (GGA) . For the relaxation of the atoms onto ground state geometry, we applied a conjugate-gradient algorithm . A special k-point sampling was used for approximating the reciprocal space Brillouin-zone (BZ) integrals. For better reliability, the optimization of the structural parameters was carried out along with successive self-consistent cycles with increasing mesh until the forces on atoms were less than 0.02 eV/Å and the stress components below 0.003 eV/Å 3 .