Along the vertical direction the frequency is given in units of terahertz (THz). Since no negative energy values are observed, the three allotropes are considered to be dynamically stable. There are 3N-3 high-energy optical modes and 3 acoustic modes. The acoustic modes start from zero energy (ω = 0) at the  point, BZ-center, up to a few terahertz. These modes correspond to the lattice rigid translation modes of the crystal (two transverse and one longitudinal).