Phonons from finite differences - Revision history

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
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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.

	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
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	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 ==

Huebsch at 08:12, 23 October 2023

2023-10-23T08:12:17Z

← Older revision		Revision as of 08:12, 23 October 2023
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	The phonon calculations using a [[Phonons:_Theory#Finite_differences\|finite differences approach]] are carried out by setting [[IBRION#ibrion56\|'''IBRION'''=5 or 6]] in the {{TAG\|INCAR}} file.		The phonon calculations using a [[Phonons:_Theory#Finite_differences\|finite differences approach]] are carried out by setting [[IBRION#ibrion56\|'''IBRION'''=5 or 6]] in the {{TAG\|INCAR}} file.
	When these ~~flags~~ are set the second-order force constants are computed using finite differences. The [[Phonons:_Theory#DynamicalMatrix\|dynamical matrix]] is constructed, diagonalized and the phonon modes and frequencies of the system are reported in the {{FILE\|OUTCAR}} file.		When these tags are set, the second-order force constants are computed using finite differences. The [[Phonons:_Theory#DynamicalMatrix\|dynamical matrix]] is constructed, diagonalized and the phonon modes and frequencies of the system are reported in the {{FILE\|OUTCAR}} file.
	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 \|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.
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	== Input ==		== Input ==

	There are two options to compute the second order force-constants using finite differences:		There are two options to compute the second-order force constants using finite differences:
	* {{TAG\|IBRION}}=5, all atoms are displaced in all three Cartesian directions, resulting in a significant computational effort even for moderately sized high-symmetry systems.		* {{TAG\|IBRION}}=5, all atoms are displaced in all three Cartesian directions, resulting in a significant computational effort even for moderately sized high-symmetry systems.
	* {{TAG\|IBRION}}=6, only symmetry inequivalent displacements are considered, and the remainder of the force constants matrix is filled using symmetry operations.		* {{TAG\|IBRION}}=6, only symmetry inequivalent displacements are considered, and the remainder of the force-constants matrix is filled using symmetry operations.

	{{TAG\|POTIM}} determines the step size. If too large values are supplied in the input file the step size defaults to 0.015 Å (starting from VASP.5.1). Expertise shows that this is a very reasonable compromise.		{{TAG\|POTIM}} determines the step size. If too large values are supplied in the input file, the step size defaults to 0.015 Å (starting from VASP.5.1). Expertise shows that this is a very reasonable compromise.

	The {{TAG\|NFREE}} tag determines how many displacements are used for each direction and ion:		The {{TAG\|NFREE}} tag determines how many displacements are used for each direction and ion:
Line 18:		Line 18:
	*{{TAG\|NFREE}}=1 uses a single displacement (this is strongly discouraged).		*{{TAG\|NFREE}}=1 uses a single displacement (this is strongly discouraged).

	If {{TAG\|ISIF}}>=3 the elastic and internal strain tensors are computed.		If {{TAG\|ISIF}}>=3, the elastic and internal strain tensors are computed.

	~~Selective~~ dynamics ~~are~~ presently only supported for {{TAG\|IBRION}}=5; in this case, only those components of the Hessian matrix are calculated for which the selective dynamics tags are set to .TRUE. in the {{FILE\|POSCAR}} file. ~~Contrary to the conventional behavior, the~~ selective dynamics ~~tags~~ refer to the Cartesian components of the Hessian matrix. For the following {{FILE\|POSCAR}} file, for instance,		The selective dynamics mode of the {{FILE\|POSCAR}} file is presently only supported for {{TAG\|IBRION}}=5; in this case, only those components of the Hessian matrix are calculated for which the selective dynamics tags are set to .TRUE. in the {{FILE\|POSCAR}} file.
			{{NB\|important\|The selective dynamics always refer to the Cartesian components of the Hessian matrix, contrary to the behavior during ionic relaxation.}} For the following {{FILE\|POSCAR}} file, for instance,

	Cubic BN		Cubic BN
Line 71:		Line 72:
	ELASTIC MODULI CONTR FROM IONIC RELAXATION (kBar)		ELASTIC MODULI CONTR FROM IONIC RELAXATION (kBar)

	The final elastic moduli including both, the contributions for distortions with rigid ions and the contributions from the ionic relaxations, are summarized at the very end:		The final elastic moduli, including both, the contributions for distortions with rigid ions and the contributions from the ionic relaxations, are summarized at the very end:

	TOTAL ELASTIC MODULI (kBar)		TOTAL ELASTIC MODULI (kBar)

	There are a few caveats to this approach: most notably the plane wave cutoff needs to be sufficiently large to converge the stress tensor. This is usually only achieved if the default cutoff is increased by roughly 30%, but it is strongly recommended to increase the cutoff systematically (e.g. in steps of 15%), until full convergence is achieved.		There are a few caveats to this approach: most notably, the plane-wave cutoff ({{TAG\|ENCUT}}) needs to be sufficiently large to converge the stress tensor. This is usually only achieved if the default cutoff is increased by roughly 30%, but it is strongly recommended to increase the cutoff systematically, (e.g., in steps of 15%), until full convergence is achieved.

	== Practical hints ==		== Practical hints ==
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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		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]].
	while varying the {{TAG\|ENCUT}}, {{TAG\|PREC}}, and '''k''' point density ({{FILE\|KPOINTS}}).
	~~This should be done in~~ the ~~unit cell {{FILE~~\|~~POSCAR}} while setting {{TAG\|IBRION}}=6 or 7 in your {{FILE\|INCAR}}~~.

	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:
	grep THz OUTCAR		grep THz OUTCAR

	To get an accurate phonon dispersion ~~it is important to~~ perform the force-constants calculation in a large enough supercell.		To get an accurate phonon dispersion, perform the force-constants calculation in a large enough supercell.
	When increasing the size of the supercell ~~it is recommended to decrease~~ the '''k''' point density in the {{FILE\|KPOINTS}} file.		When increasing the size of the supercell, we recommend decreasing the '''k'''-point density in the {{FILE\|KPOINTS}} file to yield the same resolution.
	For example, for the primitive cell of silicon, a 12x12x12 Gamma-centered '''k''' point mesh is needed to obtain accurate phonon frequencies at the Gamma point. When replicating the unit cell		For example, for the primitive cell of silicon, a 12x12x12 Gamma-centered '''k'''-point mesh is needed to obtain accurate phonon frequencies at the Gamma point. When replicating the unit cell to a 2x2x2 supercell, a 6x6x6 '''k''' point mesh will produce an equivalent sampling. For a 4x4x4 supercell, a 3x3x3 '''k''' point mesh will suffice.
	to a 2x2x2 supercell a 6x6x6 '''k''' point mesh will produce an equivalent sampling. For a 4x4x4 supercell a 3x3x3 '''k''' point mesh will suffice.

	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\|mind\|{{TAG\|IBRION}}{{=}}5, is available as of VASP.4.5, {{TAG\|IBRION}}{{=}}6 starting from VASP.5.1.}}
	{{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.}}		{{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.}}
	{{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.}}		{{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.}}

	== Related ~~Tags~~ and ~~Sections~~ ==		== Related tags and sections ==
	{{TAG\|IBRION}},		{{TAG\|IBRION}},
			{{TAG\|ISIF}},
			{{TAG\|POTIM}},
	[[Phonons: Theory]],		[[Phonons: Theory]],
	[[Phonons from density-functional-perturbation theory]]		[[Phonons from density-functional-perturbation theory]]

← 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 ==

← Older revision		Revision as of 08:12, 23 October 2023
Line 1:		Line 1:
	The phonon calculations using a [[Phonons:_Theory#Finite_differences\|finite differences approach]] are carried out by setting [[IBRION#ibrion56\|'''IBRION'''=5 or 6]] in the {{TAG\|INCAR}} file.		The phonon calculations using a [[Phonons:_Theory#Finite_differences\|finite differences approach]] are carried out by setting [[IBRION#ibrion56\|'''IBRION'''=5 or 6]] in the {{TAG\|INCAR}} file.
	When these ~~flags~~ are set the second-order force constants are computed using finite differences. The [[Phonons:_Theory#DynamicalMatrix\|dynamical matrix]] is constructed, diagonalized and the phonon modes and frequencies of the system are reported in the {{FILE\|OUTCAR}} file.		When these tags are set, the second-order force constants are computed using finite differences. The [[Phonons:_Theory#DynamicalMatrix\|dynamical matrix]] is constructed, diagonalized and the phonon modes and frequencies of the system are reported in the {{FILE\|OUTCAR}} file.
	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 \|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.
Line 7:		Line 7:
	== Input ==		== Input ==

	There are two options to compute the second order force-constants using finite differences:		There are two options to compute the second-order force constants using finite differences:
	* {{TAG\|IBRION}}=5, all atoms are displaced in all three Cartesian directions, resulting in a significant computational effort even for moderately sized high-symmetry systems.		* {{TAG\|IBRION}}=5, all atoms are displaced in all three Cartesian directions, resulting in a significant computational effort even for moderately sized high-symmetry systems.
	* {{TAG\|IBRION}}=6, only symmetry inequivalent displacements are considered, and the remainder of the force constants matrix is filled using symmetry operations.		* {{TAG\|IBRION}}=6, only symmetry inequivalent displacements are considered, and the remainder of the force-constants matrix is filled using symmetry operations.

	{{TAG\|POTIM}} determines the step size. If too large values are supplied in the input file the step size defaults to 0.015 Å (starting from VASP.5.1). Expertise shows that this is a very reasonable compromise.		{{TAG\|POTIM}} determines the step size. If too large values are supplied in the input file, the step size defaults to 0.015 Å (starting from VASP.5.1). Expertise shows that this is a very reasonable compromise.

	The {{TAG\|NFREE}} tag determines how many displacements are used for each direction and ion:		The {{TAG\|NFREE}} tag determines how many displacements are used for each direction and ion:
Line 18:		Line 18:
	*{{TAG\|NFREE}}=1 uses a single displacement (this is strongly discouraged).		*{{TAG\|NFREE}}=1 uses a single displacement (this is strongly discouraged).

	If {{TAG\|ISIF}}>=3 the elastic and internal strain tensors are computed.		If {{TAG\|ISIF}}>=3, the elastic and internal strain tensors are computed.

	~~Selective~~ dynamics ~~are~~ presently only supported for {{TAG\|IBRION}}=5; in this case, only those components of the Hessian matrix are calculated for which the selective dynamics tags are set to .TRUE. in the {{FILE\|POSCAR}} file. ~~Contrary to the conventional behavior, the~~ selective dynamics ~~tags~~ refer to the Cartesian components of the Hessian matrix. For the following {{FILE\|POSCAR}} file, for instance,		The selective dynamics mode of the {{FILE\|POSCAR}} file is presently only supported for {{TAG\|IBRION}}=5; in this case, only those components of the Hessian matrix are calculated for which the selective dynamics tags are set to .TRUE. in the {{FILE\|POSCAR}} file.
			{{NB\|important\|The selective dynamics always refer to the Cartesian components of the Hessian matrix, contrary to the behavior during ionic relaxation.}} For the following {{FILE\|POSCAR}} file, for instance,

	Cubic BN		Cubic BN
Line 71:		Line 72:
	ELASTIC MODULI CONTR FROM IONIC RELAXATION (kBar)		ELASTIC MODULI CONTR FROM IONIC RELAXATION (kBar)

	The final elastic moduli including both, the contributions for distortions with rigid ions and the contributions from the ionic relaxations, are summarized at the very end:		The final elastic moduli, including both, the contributions for distortions with rigid ions and the contributions from the ionic relaxations, are summarized at the very end:

	TOTAL ELASTIC MODULI (kBar)		TOTAL ELASTIC MODULI (kBar)

	There are a few caveats to this approach: most notably the plane wave cutoff needs to be sufficiently large to converge the stress tensor. This is usually only achieved if the default cutoff is increased by roughly 30%, but it is strongly recommended to increase the cutoff systematically (e.g. in steps of 15%), until full convergence is achieved.		There are a few caveats to this approach: most notably, the plane-wave cutoff ({{TAG\|ENCUT}}) needs to be sufficiently large to converge the stress tensor. This is usually only achieved if the default cutoff is increased by roughly 30%, but it is strongly recommended to increase the cutoff systematically, (e.g., in steps of 15%), until full convergence is achieved.

	== Practical hints ==		== Practical hints ==
Line 83:		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		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]].
	while varying the {{TAG\|ENCUT}}, {{TAG\|PREC}}, and '''k''' point density ({{FILE\|KPOINTS}}).
	~~This should be done in~~ the ~~unit cell {{FILE~~\|~~POSCAR}} while setting {{TAG\|IBRION}}=6 or 7 in your {{FILE\|INCAR}}~~.

	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:
	grep THz OUTCAR		grep THz OUTCAR

	To get an accurate phonon dispersion ~~it is important to~~ perform the force-constants calculation in a large enough supercell.		To get an accurate phonon dispersion, perform the force-constants calculation in a large enough supercell.
	When increasing the size of the supercell ~~it is recommended to decrease~~ the '''k''' point density in the {{FILE\|KPOINTS}} file.		When increasing the size of the supercell, we recommend decreasing the '''k'''-point density in the {{FILE\|KPOINTS}} file to yield the same resolution.
	For example, for the primitive cell of silicon, a 12x12x12 Gamma-centered '''k''' point mesh is needed to obtain accurate phonon frequencies at the Gamma point. When replicating the unit cell		For example, for the primitive cell of silicon, a 12x12x12 Gamma-centered '''k'''-point mesh is needed to obtain accurate phonon frequencies at the Gamma point. When replicating the unit cell to a 2x2x2 supercell, a 6x6x6 '''k''' point mesh will produce an equivalent sampling. For a 4x4x4 supercell, a 3x3x3 '''k''' point mesh will suffice.
	to a 2x2x2 supercell a 6x6x6 '''k''' point mesh will produce an equivalent sampling. For a 4x4x4 supercell a 3x3x3 '''k''' point mesh will suffice.

	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\|mind\|{{TAG\|IBRION}}{{=}}5, is available as of VASP.4.5, {{TAG\|IBRION}}{{=}}6 starting from VASP.5.1.}}
	{{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.}}		{{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.}}
	{{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.}}		{{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.}}

	== Related ~~Tags~~ and ~~Sections~~ ==		== Related tags and sections ==
	{{TAG\|IBRION}},		{{TAG\|IBRION}},
			{{TAG\|ISIF}},
			{{TAG\|POTIM}},
	[[Phonons: Theory]],		[[Phonons: Theory]],
	[[Phonons from density-functional-perturbation theory]]		[[Phonons from density-functional-perturbation theory]]