$C_2/C_1$
$C_2/C_1$ 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.