View source for Computing the phonon dispersion and DOS

After computing the force constants using the [[Phonons_from_finite_differences|finite differences]] or [[Phonons_from_density-functional-perturbation_theory|density functional perturbation theory]] approaches it is possible to compute the phonon dispersion using the primitive cell 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 [[Phonons_from_finite_differences|finite differences]] with ({{TAG|IBRION}}=5, 6)
# Using [[Phonons_from_density-functional-perturbation_theory|density functional perturbation theory]] with ({{TAG|IBRION}}=7, 8)

== Computing the phonon dispersion ==
To plot the phonon dispersion the {{FILE | QPOINTS}} file must be present, contain a q-points path, and the tag {{TAG | LPHON_DISPERSION}}=.TRUE. 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 correct phonon dispersion.
This is activated by setting {{TAG | LPHON_POLAR}}=.TRUE. and supplying the static dielectric tensor ({{TAG | PHON_DIELECTRIC}}) and the Born-effective charges ({{TAG | PHON_BORN_CHARGES}}) which can be obtained from a separate VASP run using the {{TAG|LEPSILON}} tag. The long-range part is evaluated by an Ewald sum. To use the Ewald summation the user has to set a plane wave cutoff radius ({{TAG | PHON_G_CUTOFF}}).
The amount of information written to the {{FILE | OUTCAR}} file can be tuned using the ({{TAG|PHON_NWRITE}} tag).

== Computing the density of states ==

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

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

==Related tags and articles==
{{FILE| QPOINTS}},
{{TAG | LPHON_DISPERSION}},
{{TAG | PHON_NWRITE}},
{{TAG | LPHON_POLAR}},
{{TAG | PHON_DIELECTRIC}},
{{TAG | PHON_BORN_CHARGES}},
{{TAG | PHON_G_CUTOFF}}

[[Category:Phonons]][[Category:Howto]]