  Structural stability and vibrational properties of KN$_3$ have been studied
under pressure using the first principles calculations based on the density
functional theory (DFT) as well as with semi empirical dispersion correction to
the DFT to treat van der Waals interactions. The structural properties
calculated through the dispersion correction are in good agreement with the
experimental results when compared with the standard density functional theory.
The lattice mode and the azide ion bending mode frequencies are found to be
well reproduced by our calculations. As pressure increases, the E$_g$ mode gets
softened as observed in the experiment. Despite the softening of E$_g$ mode,
the enhancement of intramolecular interactions keeps the lattice stable.
