Closer examination allowed a relationship to be established with silicon carbide. SiC is known to have 4H, 6H and 8H polytypes characterized by 4, 6 and 8 layers along the hexagonal vertical direction (cf. Figs. 2b, 2c, 2d for BN). Then, the currently calculated BN structure is a 4H polytype crystallizing in space group P6 3 mc. In this context, hypothetical 6H and 8H BN polytypes were calculated. The second and third data columns of Table give the equilibrium ground state hexagonal lattice parameters a and c, and the atomic positions of the proposed 6H and 8H polytypes. The corresponding structures are shown in Figs. and. The last row of Table shows that all three polytypes are largely cohesive and have close E coh /atom values. Topology analysis led to an original cfd topology; 6-layered SiC polytype/chcchc (c and h indicate layer nature) for 6H BN, and "8-layered SiC polytype/chccchcc"-type for 8H BN, i.e. different from cfc, which defines C 8 and 4H BN. However, all BN polytypes are structurally related.