Along the horizontal direction, the bands run along the main lines of the tetragonal Brillouin zone (reciprocal k-space). The vertical direction shows the frequencies given in units of terahertz (THz). Since no negative frequency magnitudes are observed in all three panels all structures are considered as dynamically stable. There are 3N-3 optical modes at higher energy than three acoustic modes which start from zero energy (ω = 0) at the  point, center of the Brillouin Zone, up to a few Terahertz. The 3 acoustic modes correspond to the lattice rigid translation modes of the crystal (two transverse and one longitudinal). The remaining bands correspond to the acoustic modes and culminating at ω ~ 40 THz in C 4 , a magnitude observed for diamond by Raman spectroscopy , and only up to 34 in less stable C 3 (cf.