A relevant criterion of phase stability can be obtained from the dynamic properties derived from the phonon modes. The phonon energies E = ћω (where ћ is the reduced Planck constant and ω is phonon frequency) were then calculated, and the corresponding band structures were plotted along the Brillouin zone in the reciprocal space. Phonons have been calculated for C 8 , BC 2 N, and 4H BN; the latter representing all three BN polytypes. Fig. shows the corresponding band structures. 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. In Fig , there are no negative frequencies, confirming that C 8 carbon allotrope (Fig. ), hexagonal BN polytypes (Fig. ) and BC 2 N (Fig. ) are dynamically stable. In C 8 (Fig. ), the highest band is found at ω ~ 40 THz, a value observed for diamond by Raman spectroscopy .