The bands develop along the main lines (horizontal direction) of the hexagonal Brillouin zone (reciprocal k-space). The vertical direction shows the frequencies ω, which are given in terahertz (THz). There are 3N phonon total modes with 3 acoustic modes starting from zero frequency (ω = 0 at the  point, center of the Brillouin zone), up to a few terahertz and 3N-3 optical modes at frequencies higher than three. The three acoustic modes correspond to the lattice rigid translation modes with two transverse modes and one longitudinal mode. The remaining bands correspond to the optical modes. In all three subfigures there are no negative frequencies, and the corresponding carbon allotrope and two binary phases are dynamically stable. The latter indicates that these phases, once synthesized, can exist at ambient conditions. In qtz C 6 , the highest band culminates in the vicinity of ω ~ 40 THz, a value that has been observed for diamond by Raman spectroscopy . Binary compounds are characterized by lower energy bands. ....