The atoms were relaxed to the ground state structures using the conjugate gradient algorithm according to Press et al. . The Blöchl tetrahedron method with corrections according to the Methfessel and Paxton scheme was used for geometry optimization and energy calculations, respectively. Brillouin-zone integrals were approximated by a special k-point sampling according to Monkhorst and Pack . Structural parameters were optimized until atomic forces were below 0.02 eV/Å and all stress components were < 0.003 eV/Å 3 . The calculations were converged at an energy cutoff of 400 eV for the plane-wave basis set in terms of the k-point integration in the reciprocal space from k x (6)×k y (6)×k z (6) up to k x ×k y ×k z for zb-BX and k x (6)×k y (6)×k z up to k x ×k y ×k z for qtz BX to obtain a final convergence and relaxation to zero strains. In the post-processing of the ground state electronic structures, the charge density projections were operated on the lattice sites.