A further qualitative illustration of the different types of hybridization is obtained from the projections of the charge densities. Figs 2a, 2b and 2c, which reproduce the crystal structure sketches of Fig. , highlight the volumes shown in yellow. In diamond-like C 18 (Fig. ), the sp 3 -type hybridization is clearly observed from the tetrahedral volumes around the C atoms. Like diamond, the rhombohedral 9R polytype is a perfectly covalent system with similar physical properties (mechanical, dynamic, etc.) and electronic structure as shown below. Changes are observed in the hybrid ene-C 21 (Fig. ), where the sp 3