Phonon dispersion relation of a bcc lattice

$\omega\sqrt{\frac{m}{C_1}}$
bcc crystal

$C_2/C_1$

$C_2/C_1$ is the relative value of $C_2$, the spring constant of the next-nearest neighbours, to $C_1$, the spring constant to the nearest neighbours.

For $C_2/C_1 = 0$ with only the nearest neighbours considered one of the transverse modes in the Γ-N direction always has a frequency of zero. By considering the next-nearest neighbors, the structure is not restricted to oscillate in one direction any more. Another notable difference from experimental data is the overlap of all modes in the Γ-H direction.