From the densities, the diamond-like C 4 structure is found to have the highest density, i.e. ρ(C 4 ) = 3.50 g/cm 3 versus ρ(C 9 ) = 2.92 g/cm 3 , and ρ(C 12 ) = 2.97 g/cm 3 . The reason is that in the two novel allotropes the connection between the tetrahedra is not as regular as in diamond or C 4 .