Phonons from finite differences - Revision history

Schlipf: Add backlink to How to handle imaginary phonon modes (via VASP Copilot skill sync)

2026-05-08T15:10:09Z

Add backlink to How to handle imaginary phonon modes (via VASP Copilot skill sync)

← Older revision		Revision as of 15:10, 8 May 2026
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	Although VASP.4.6 supports {{TAG\|IBRION}}{{=}}5-6, VASP.4.6 does not change the set of '''k''' points automatically (often the displaced configurations require a different '''k'''-point grid). Hence, the finite difference routine might yield incorrect results in VASP.4.6.		Although VASP.4.6 supports {{TAG\|IBRION}}{{=}}5-6, VASP.4.6 does not change the set of '''k''' points automatically (often the displaced configurations require a different '''k'''-point grid). Hence, the finite difference routine might yield incorrect results in VASP.4.6.

	If you obtain imaginary frequencies when you are not expecting them (i.e., at a [[:Category:Transition states \| transition state]]), then this is likely an issue with convergence. You should try increasing the cell size or [[KPOINTS \| k-points mesh]]. If this does not work, consider the [[ISMEAR \| smearing parameter]], increasing the precision with the {TAG\|PREC}} tag, or the {{TAG\|ENCUT}}.		If you obtain imaginary frequencies when you are not expecting them (i.e., at a [[:Category:Transition states \| transition state]]), then this is likely an issue with convergence. You should try increasing the cell size or [[KPOINTS \| k-points mesh]]. If this does not work, consider the [[ISMEAR \| smearing parameter]], increasing the precision with the {TAG\|PREC}} tag, or the {{TAG\|ENCUT}}. See [[How to handle imaginary phonon modes]] for a step-by-step guide to identifying and resolving soft modes.

	== Related tags and sections ==		== Related tags and sections ==
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	[[Phonons: Theory]]		[[Phonons: Theory]]

	[[Phonons from density-functional-perturbation theory]], [[Computing the phonon dispersion and DOS]]		[[Phonons from density-functional-perturbation theory]], [[Computing the phonon dispersion and DOS]], [[How to handle imaginary phonon modes]]

	== References==		== References==

Csheldon: /* Practical hints */

2026-02-09T10:39:51Z

Practical hints

← Older revision		Revision as of 10:39, 9 February 2026
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	In some older versions (pre VASP.5.1), {{TAG\|NSW}} (number of ionic steps) must be set to 1 in the {{FILE\|INCAR}} file, since {{TAG\|NSW}}{{=}}0 sets the {{TAG\|IBRION}}{{=}}−1 regardless of the value supplied in the {{FILE\|INCAR}} file.		In some older versions (pre VASP.5.1), {{TAG\|NSW}} (number of ionic steps) must be set to 1 in the {{FILE\|INCAR}} file, since {{TAG\|NSW}}{{=}}0 sets the {{TAG\|IBRION}}{{=}}−1 regardless of the value supplied in the {{FILE\|INCAR}} file.
	Although VASP.4.6 supports {{TAG\|IBRION}}{{=}}5-6, VASP.4.6 does not change the set of '''k''' points automatically (often the displaced configurations require a different '''k'''-point grid). Hence, the finite difference routine might yield incorrect results in VASP.4.6.		Although VASP.4.6 supports {{TAG\|IBRION}}{{=}}5-6, VASP.4.6 does not change the set of '''k''' points automatically (often the displaced configurations require a different '''k'''-point grid). Hence, the finite difference routine might yield incorrect results in VASP.4.6.

			If you obtain imaginary frequencies when you are not expecting them (i.e., at a [[:Category:Transition states \| transition state]]), then this is likely an issue with convergence. You should try increasing the cell size or [[KPOINTS \| k-points mesh]]. If this does not work, consider the [[ISMEAR \| smearing parameter]], increasing the precision with the {TAG\|PREC}} tag, or the {{TAG\|ENCUT}}.

	== Related tags and sections ==		== Related tags and sections ==

Csheldon: /* Practical hints */

2026-02-09T10:05:11Z

Practical hints

← Older revision		Revision as of 10:05, 9 February 2026
Line 100:		Line 100:
	In some older versions (pre VASP.5.1), {{TAG\|NSW}} (number of ionic steps) must be set to 1 in the {{FILE\|INCAR}} file, since {{TAG\|NSW}}{{=}}0 sets the {{TAG\|IBRION}}{{=}}−1 regardless of the value supplied in the {{FILE\|INCAR}} file.		In some older versions (pre VASP.5.1), {{TAG\|NSW}} (number of ionic steps) must be set to 1 in the {{FILE\|INCAR}} file, since {{TAG\|NSW}}{{=}}0 sets the {{TAG\|IBRION}}{{=}}−1 regardless of the value supplied in the {{FILE\|INCAR}} file.
	Although VASP.4.6 supports {{TAG\|IBRION}}{{=}}5-6, VASP.4.6 does not change the set of '''k''' points automatically (often the displaced configurations require a different '''k'''-point grid). Hence, the finite difference routine might yield incorrect results in VASP.4.6.		Although VASP.4.6 supports {{TAG\|IBRION}}{{=}}5-6, VASP.4.6 does not change the set of '''k''' points automatically (often the displaced configurations require a different '''k'''-point grid). Hence, the finite difference routine might yield incorrect results in VASP.4.6.

	Bear in mind that the choice of exchange-correlation functional (e.g., hybrid vs GGA functionals) can have a big impact on the [[Born effective charges]] and the [[:Category:Dielectric_properties#Dielectric_function \| dielectric function]], and thereby the LO-TO splitting.

	== Related tags and sections ==		== Related tags and sections ==

Csheldon: /* Practical hints */

2026-02-09T10:02:48Z

Practical hints

← Older revision		Revision as of 10:02, 9 February 2026
Line 101:		Line 101:
	Although VASP.4.6 supports {{TAG\|IBRION}}{{=}}5-6, VASP.4.6 does not change the set of '''k''' points automatically (often the displaced configurations require a different '''k'''-point grid). Hence, the finite difference routine might yield incorrect results in VASP.4.6.		Although VASP.4.6 supports {{TAG\|IBRION}}{{=}}5-6, VASP.4.6 does not change the set of '''k''' points automatically (often the displaced configurations require a different '''k'''-point grid). Hence, the finite difference routine might yield incorrect results in VASP.4.6.

	Bear in mind that the choice of exchange-correlation functional can have a big impact on the [[Born effective charges]] and the [[:Category:Dielectric_properties#Dielectric_function \| dielectric function]], and thereby the LO-TO splitting.		Bear in mind that the choice of exchange-correlation functional (e.g., hybrid vs GGA functionals) can have a big impact on the [[Born effective charges]] and the [[:Category:Dielectric_properties#Dielectric_function \| dielectric function]], and thereby the LO-TO splitting.

	== Related tags and sections ==		== Related tags and sections ==

Csheldon: /* Practical hints */

2026-02-09T10:02:05Z

Practical hints

← Older revision		Revision as of 10:02, 9 February 2026
Line 100:		Line 100:
	In some older versions (pre VASP.5.1), {{TAG\|NSW}} (number of ionic steps) must be set to 1 in the {{FILE\|INCAR}} file, since {{TAG\|NSW}}{{=}}0 sets the {{TAG\|IBRION}}{{=}}−1 regardless of the value supplied in the {{FILE\|INCAR}} file.		In some older versions (pre VASP.5.1), {{TAG\|NSW}} (number of ionic steps) must be set to 1 in the {{FILE\|INCAR}} file, since {{TAG\|NSW}}{{=}}0 sets the {{TAG\|IBRION}}{{=}}−1 regardless of the value supplied in the {{FILE\|INCAR}} file.
	Although VASP.4.6 supports {{TAG\|IBRION}}{{=}}5-6, VASP.4.6 does not change the set of '''k''' points automatically (often the displaced configurations require a different '''k'''-point grid). Hence, the finite difference routine might yield incorrect results in VASP.4.6.		Although VASP.4.6 supports {{TAG\|IBRION}}{{=}}5-6, VASP.4.6 does not change the set of '''k''' points automatically (often the displaced configurations require a different '''k'''-point grid). Hence, the finite difference routine might yield incorrect results in VASP.4.6.

			Bear in mind that the choice of exchange-correlation functional can have a big impact on the [[Born effective charges]] and the [[:Category:Dielectric_properties#Dielectric_function \| dielectric function]], and thereby the LO-TO splitting.

	== Related tags and sections ==		== Related tags and sections ==

Huebsch: /* Related tags and sections */

2025-11-06T09:00:41Z

Related tags and sections

← Older revision		Revision as of 09:00, 6 November 2025
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	{{TAG\|ISIF}},		{{TAG\|ISIF}},
	{{TAG\|POTIM}},		{{TAG\|POTIM}},
	[[Phonons: Theory]],
	[[Phonons from density-functional-perturbation theory]]		[[Phonons: Theory]]

			[[Phonons from density-functional-perturbation theory]], [[Computing the phonon dispersion and DOS]]

	== References==		== References==

Mani: Fix: Phonon eigenvectors are written in Cartesian coordinates and not in direct coordinates.

2024-07-31T12:03:45Z

Fix: Phonon eigenvectors are written in Cartesian coordinates and not in direct coordinates.

← Older revision		Revision as of 12:03, 31 July 2024
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	The first number is the label of the normal mode. If this number is followed by ''f'' it is a purely real mode, stating the mode is vibrationally stable. Otherwise, if it is followed by ''f/i'', the mode is an imaginary mode ("soft mode"). These labels are followed by the eigenfrequency of the mode in different units.		The first number is the label of the normal mode. If this number is followed by ''f'' it is a purely real mode, stating the mode is vibrationally stable. Otherwise, if it is followed by ''f/i'', the mode is an imaginary mode ("soft mode"). These labels are followed by the eigenfrequency of the mode in different units.

	The following table labeled by (''x,y,z,dx,dy,dz'') contains the Cartesian positions of the atoms and the normalized eigenvectors of the eigenmodes in ~~direct~~ coordinates.		The following table labeled by (''x,y,z,dx,dy,dz'') contains the Cartesian positions of the atoms and the normalized eigenvectors of the eigenmodes in Cartesian coordinates.

	There should be 3<math>N</math> normal modes, where <math>N</math> is the number of atoms in the supercell ({{TAG\|POSCAR}}). The modes are ordered in descending order with respect to the eigenfrequency. The last three modes are the translational modes (they are usually disregarded).		There should be 3<math>N</math> normal modes, where <math>N</math> is the number of atoms in the supercell ({{TAG\|POSCAR}}). The modes are ordered in descending order with respect to the eigenfrequency. The last three modes are the translational modes (they are usually disregarded).

Huebsch: /* Input */

2023-10-23T09:41:57Z

Input

← Older revision		Revision as of 09:41, 23 October 2023
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	atom 2 is displaced in the ''x''-direction only, and only the ''x''-component of the second atom of the Hessian matrix is calculated.		atom 2 is displaced in the ''x''-direction only, and only the ''x''-component of the second atom of the Hessian matrix is calculated.

			If {{TAG\|LEPSILON}}=.TRUE. or {{TAG\|LCALCEPS}}=.TRUE., additional dielectric properties are computed.

	== Output ==		== Output ==

Huebsch at 09:30, 23 October 2023

2023-10-23T09:30:15Z

← Older revision		Revision as of 09:30, 23 October 2023
Line 3:		Line 3:
	If {{TAG\|ISIF}}>=3, the internal strain tensors are computed as well.		If {{TAG\|ISIF}}>=3, the internal strain tensors are computed as well.
	{{NB\|mind\|Only zone-center (Γ-point) frequencies are calculated.}}		{{NB\|mind\|Only zone-center (Γ-point) frequencies are calculated.}}
	It is possible to [[~~Computing_the_phonon_dispersion~~ \|obtain the phonon dispersion at different '''q''' points]] by computing the second-order force constants on a sufficiently large supercell and Fourier interpolating the dynamical matrices in the unit cell.		It is possible to [[Computing_the_phonon_dispersion_and_DOS \|obtain the phonon dispersion at different '''q''' points]] by computing the second-order force constants on a sufficiently large supercell and Fourier interpolating the dynamical matrices in the unit cell.

	== Input ==		== Input ==

Huebsch at 08:25, 23 October 2023

2023-10-23T08:25:41Z

← Older revision		Revision as of 08:25, 23 October 2023
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	The {{TAG\|ADDGRID}}=TRUE should '''not''' be set without careful testing.		The {{TAG\|ADDGRID}}=TRUE should '''not''' be set without careful testing.

	A practical way to check for convergence is to monitor the Γ point ('''q'''=0) optical mode frequencies while varying the {{TAG\|ENCUT}}, {{TAG\|PREC}}, and '''k''' point density ({{FILE\|KPOINTS}}). Additionally, compare the result to [[Phonons from density-functional-perturbation theory\|phonons from density-functional-perturbation theory]].		A practical way to check for convergence is to monitor the Γ point ('''q'''=0) optical mode frequencies while varying the {{TAG\|ENCUT}}, {{TAG\|PREC}}, and '''k''' point density ({{FILE\|KPOINTS}}). Additionally, compare the result to [[Phonons from density-functional-perturbation theory\|phonons from density-functional-perturbation theory (DFPT)]].

	To get the phonon frequencies quickly on the command line, simply type the following:		To get the phonon frequencies quickly on the command line, simply type the following:
Line 94:		Line 94:

	It is possible to use phonopy{{cite\|phonopy}} to post-process the results of a finite differences calculation done with VASP.{{cite\|phonopy_dfpt}}		It is possible to use phonopy{{cite\|phonopy}} to post-process the results of a finite differences calculation done with VASP.{{cite\|phonopy_dfpt}}
	{{NB\|~~mind~~\|{{TAG\|IBRION}}{{=}}5, is available as of VASP.4.5, {{TAG\|IBRION}}{{=}}6 starting from VASP.5.1.}}		{{NB\|tip\|In contrast to [[Phonons from density-functional-perturbation theory\|computing phonons within DFPT]], the finite difference approach can be used in combination with any [[Exchange-correlation functional]].}}
	~~{{NB\|mind\|~~In some older versions (pre VASP.5.1), {{TAG\|NSW}} (number of ionic steps) must be set to 1 in the {{FILE\|INCAR}} file, since {{TAG\|NSW}}{{=}}0 sets the {{TAG\|IBRION}}{{=}}−1 regardless of the value supplied in the {{FILE\|INCAR}} file.}}		{{TAG\|IBRION}}{{=}}5, is available as of VASP.4.5, {{TAG\|IBRION}}{{=}}6 starting from VASP.5.1.
	~~{{NB\|mind\|~~Although VASP.4.6 supports {{TAG\|IBRION}}{{=}}5-6, VASP.4.6 does not change the set of '''k''' points automatically (often the displaced configurations require a different '''k'''-point grid). Hence, the finite difference routine might yield incorrect results in VASP.4.6.}}		In some older versions (pre VASP.5.1), {{TAG\|NSW}} (number of ionic steps) must be set to 1 in the {{FILE\|INCAR}} file, since {{TAG\|NSW}}{{=}}0 sets the {{TAG\|IBRION}}{{=}}−1 regardless of the value supplied in the {{FILE\|INCAR}} file.
			Although VASP.4.6 supports {{TAG\|IBRION}}{{=}}5-6, VASP.4.6 does not change the set of '''k''' points automatically (often the displaced configurations require a different '''k'''-point grid). Hence, the finite difference routine might yield incorrect results in VASP.4.6.

	== Related tags and sections ==		== Related tags and sections ==

← Older revision		Revision as of 08:25, 23 October 2023
Line 84:		Line 84:
	The {{TAG\|ADDGRID}}=TRUE should '''not''' be set without careful testing.		The {{TAG\|ADDGRID}}=TRUE should '''not''' be set without careful testing.

	A practical way to check for convergence is to monitor the Γ point ('''q'''=0) optical mode frequencies while varying the {{TAG\|ENCUT}}, {{TAG\|PREC}}, and '''k''' point density ({{FILE\|KPOINTS}}). Additionally, compare the result to [[Phonons from density-functional-perturbation theory\|phonons from density-functional-perturbation theory]].		A practical way to check for convergence is to monitor the Γ point ('''q'''=0) optical mode frequencies while varying the {{TAG\|ENCUT}}, {{TAG\|PREC}}, and '''k''' point density ({{FILE\|KPOINTS}}). Additionally, compare the result to [[Phonons from density-functional-perturbation theory\|phonons from density-functional-perturbation theory (DFPT)]].

	To get the phonon frequencies quickly on the command line, simply type the following:		To get the phonon frequencies quickly on the command line, simply type the following:
Line 94:		Line 94:

	It is possible to use phonopy{{cite\|phonopy}} to post-process the results of a finite differences calculation done with VASP.{{cite\|phonopy_dfpt}}		It is possible to use phonopy{{cite\|phonopy}} to post-process the results of a finite differences calculation done with VASP.{{cite\|phonopy_dfpt}}
	{{NB\|~~mind~~\|{{TAG\|IBRION}}{{=}}5, is available as of VASP.4.5, {{TAG\|IBRION}}{{=}}6 starting from VASP.5.1.}}		{{NB\|tip\|In contrast to [[Phonons from density-functional-perturbation theory\|computing phonons within DFPT]], the finite difference approach can be used in combination with any [[Exchange-correlation functional]].}}
	~~{{NB\|mind\|~~In some older versions (pre VASP.5.1), {{TAG\|NSW}} (number of ionic steps) must be set to 1 in the {{FILE\|INCAR}} file, since {{TAG\|NSW}}{{=}}0 sets the {{TAG\|IBRION}}{{=}}−1 regardless of the value supplied in the {{FILE\|INCAR}} file.}}		{{TAG\|IBRION}}{{=}}5, is available as of VASP.4.5, {{TAG\|IBRION}}{{=}}6 starting from VASP.5.1.
	~~{{NB\|mind\|~~Although VASP.4.6 supports {{TAG\|IBRION}}{{=}}5-6, VASP.4.6 does not change the set of '''k''' points automatically (often the displaced configurations require a different '''k'''-point grid). Hence, the finite difference routine might yield incorrect results in VASP.4.6.}}		In some older versions (pre VASP.5.1), {{TAG\|NSW}} (number of ionic steps) must be set to 1 in the {{FILE\|INCAR}} file, since {{TAG\|NSW}}{{=}}0 sets the {{TAG\|IBRION}}{{=}}−1 regardless of the value supplied in the {{FILE\|INCAR}} file.
			Although VASP.4.6 supports {{TAG\|IBRION}}{{=}}5-6, VASP.4.6 does not change the set of '''k''' points automatically (often the displaced configurations require a different '''k'''-point grid). Hence, the finite difference routine might yield incorrect results in VASP.4.6.

	== Related tags and sections ==		== Related tags and sections ==