Computing the phonon dispersion and DOS: Difference between revisions

Revision as of 09:00, 23 October 2023

After computing the force constants using the finite differences or 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.

Phonon dispersion: Step-by-step instructions

Step 1: Compute the force constants.

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

Using finite differences with (IBRION=5, 6)
Using density functional perturbation theory with (IBRION=7, 8).

Step 2: Provide a q-points along a high-symmetry path.

Create a QPOINTS file containing a q-points path at which the phonon dispersion will be computed.

Step 3: Compute the phonon dispersion.

To plot the phonon dispersion, the tag LPHON_DISPERSION=.TRUE. must be set in the INCAR file.

If the material is polar, i.e., has more than one different atom type without being centro-symmetric, the long-range dipole-dipole interaction has to be treated by Ewald summation. This is achieved by setting LPHON_POLAR=.TRUE., supplying the static dielectric tensor (PHON_DIELECTRIC) and the Born-effective charges (PHON_BORN_CHARGES). The values for these have to be obtained from a separate VASP run setting the LEPSILON or LCALCEPS INCAR tags. Additionally, the user might specify a reciprocal space cutoff radius (PHON_G_CUTOFF) for the Ewald summation.

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

Mind: The computation of the phonon dispersion after a finite differences calculation is only supported for IBRION=6. For IBRION=5,7 or 8 the calculation has to be done in two steps as explained in LPHON_READ_FORCE_CONSTANTS.

Phonon density of states: Step-by-step instructions

Step 1: Compute the force constants

Same as above.

Step 2: Specify a uniform q-point mesh

Create a QPOINTS file that specifies a sufficiently dense, uniform q-point mesh.

Step 3: Compute the density of states

Set LPHON_DISPERSION=.TRUE. in the INCAR 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 tetrahedron method set PHON_DOS=2.

@@ Line 1: / Line 1: @@
-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.
+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 ==
+== Phonon dispersion: Step-by-step instructions ==
+=== Step 1: Compute the 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)
+# Using [[Phonons_from_density-functional-perturbation_theory|density functional perturbation theory]] with ({{TAG|IBRION}}=7, 8).
+=== Step 2: Provide a '''q'''-points along a high-symmetry path. ===
+Create a {{FILE|QPOINTS}} file containing a '''q'''-points path at which the phonon dispersion will be computed.
-== Computing the phonon dispersion ==
+=== Step 3: Compute the phonon dispersion. ===
-To plot the phonon dispersion the tag {{TAG | LPHON_DISPERSION}}=.TRUE. must be set in the {{FILE|INCAR}} file.
+To plot the phonon dispersion, the tag {{TAG|LPHON_DISPERSION}}=.TRUE. must be set in the {{FILE|INCAR}} file.
-Additionally, the {{FILE | QPOINTS}} file must be present containing a '''q''' points path at which the phonon dispersion will be computed.
-If the material is polar (i.e. has more than one different atom type) the long-range dipole-dipole interaction has to be treated by an [[Phonons:_Theory#Long-range_interatomic_force_constants_.28LO-TO_splitting.29|Ewald summation]].
+If the material is polar, i.e., has more than one different atom type without being centro-symmetric, the long-range dipole-dipole interaction has to be treated by [[Phonons:_Theory#Long-range_interatomic_force_constants_.28LO-TO_splitting.29|Ewald summation]].
-This is achieved by setting {{TAG | LPHON_POLAR}}=.TRUE., supplying the static dielectric tensor ({{TAG | PHON_DIELECTRIC}}) and the Born-effective charges ({{TAG | PHON_BORN_CHARGES}}).
+This is achieved by setting {{TAG|LPHON_POLAR}}=.TRUE., supplying the static dielectric tensor ({{TAG|PHON_DIELECTRIC}}) and the Born-effective charges ({{TAG|PHON_BORN_CHARGES}}).
 The values for these have to be obtained from a separate VASP run setting the {{TAG|LEPSILON}} or {{TAG|LCALCEPS}} INCAR tags.
-Additionally, the user might specify a reciprocal space cutoff radius ({{TAG | PHON_G_CUTOFF}}) for the Ewald summation.
+Additionally, the user might specify a reciprocal space cutoff radius ({{TAG|PHON_G_CUTOFF}}) for the Ewald summation.
 The amount of information written to the {{FILE | OUTCAR}} file can be tuned using the ({{TAG|PHON_NWRITE}} tag).
 {{NB|mind|The computation of the phonon dispersion after a finite differences calculation is only supported for {{TAG|IBRION}}{{=}}6. For {{TAG|IBRION}}{{=}}5,7 or 8 the calculation has to be done in two steps as explained in {{TAG|LPHON_READ_FORCE_CONSTANTS}}.}}
-== Computing the density of states ==
+== Phonon density of states: Step-by-step instructions ==
+=== Step 1: Compute the force constants ===
+Same as above.
+=== Step 2: Specify a uniform '''q'''-point mesh ===
+Create a {{FILE|QPOINTS}} file that specifies a sufficiently dense, uniform '''q'''-point mesh.
+=== Step 3: Compute the density of states ===
-To compute the phonon density of states a sufficiently dense q-point mesh should be specified in the {{FILE|QPOINTS}} file and {{TAG | LPHON_DISPERSION}}=.TRUE. must be set in the {{FILE|INCAR}} file.. The density of states is computed between
+Set {{TAG | LPHON_DISPERSION}}=.TRUE. in the {{FILE|INCAR}} 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}}.
+<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.
+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==