which is characterized by a single four-fold atomic position for carbon at ⅓, ⅔, z with a small (0.06275) z-value along the hexagonal vertical axis. Changing z to 0 gives a 2D graphite-like carbon structure (Fig. ) with hcb topology. Then, the puckering of the graphite-like layers is a key factor in the transition from 2D to 3D structure. Here we model the 2D  3D transformation by inserting a carbon atom between the carbon layers, as shown by the white sphere in Fig. .