Computing the phonon dispersion and DOS

After computing the force constants using the finite differences or density functional perturbation theory approaches it is possible to compute the phonon dispersion as well as the phonon density of states.

Computing force constants

There are two possible approaches for computing the force constants and then building the dynamical matrix:

Using finite differences of the forces (IBRION=5, 6)
Using density functional perturbation theory (IBRION=7, 8)

Computing the phonon dispersion

To plot the phonon dispersion the QPOINTS file must be present, contain a path, and the LPHON_DISPERSION=.TRUE. tag must be set.

If the material is polar (i.e. has more than one different atom type) a special treatment of the long-range dipole-dipole interaction is required to obtain a smooth phonon dispersion. This is activated by setting LPHON_POLAR=.TRUE. and supplying the static dielectric tensor (PHON_DIELECTRIC) and the Born-effective charges (PHON_BORN_CHARGES) which can be obtained on a separate VASP run using the LEPSILON tag. The long-range part is evaluated using an Ewald sum with the number of G-vectors in reciprocal space being determined using a cutoff length (PHON_G_CUTOFF).

The amount of information written to the OUTCAR file can be tuned using the (PHON_NWRITE tag)

Computing the density of states

To compute the phonon density of states a regular mesh should be specified in the QPOINTS file. The density of states is computed between $[\omega_{\text{min}}-5\sigma,\omega_{\text{max}}+5\sigma]$ with $\omega_{\text{min}}$ and $\omega_{\text{max}}$ the lowest and highest phonon frequency and $\sigma$ the broadening PHON_SIGMA.

The number of energy points in this energy range is specified by the PHON_NEDOS tag. To use a gaussian smearing method for the computation of the DOS set PHON_DOS=1 to use the the tetrahedron method set PHON_DOS=2.

Computing force constants

Computing the phonon dispersion

Computing the density of states

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