All devised structural setups have been subjected to unconstrained geometry relaxations of atomic positions and lattice constants down to the ground states characterized by minimum energies. The protocol consists of successive iterative cycles with increasing precision of the Brillouin zone (BZ) k-mesh in the reciprocal lattice. The calculations were performed using the DFT-based plane-wave Vienna Ab initio Simulation Package (VASP) . The projector augmented wave (PAW) method was used for the atomic potentials . The exchange X and correlation effects (XC) were treated within a Generalized Gradient Approximation (GGA) scheme . Test calculations with the hybrid functional HSE06 did not lead to better results than GGA. The relaxation of the atoms to the ground state geometry was performed using a conjugate-gradient algorithm . Blöchl tetrahedron method with corrections according to Methfessel and Paxton scheme was used for geometry optimization and energy calculations. A special k-point sampling was used to approximate the BZ reciprocal space integrals. For better reliability, the optimization of the structural parameters was carried out along with successive self-consistent cycles with increasing k-mesh until the forces on atoms were less than 0.02 eV/Å and the stress components were less than 0.003 eV/Å 3 .