ISIF: Difference between revisions

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{{TAGDEF|ISIF|0 {{!}} 1 {{!}} 2 {{!}} 3 {{!}} 4 {{!}} 5 {{!}} 6 {{!}} 7}}
{{TAGDEF|ISIF|0 {{!}} 1 {{!}} 2 {{!}} 3 {{!}} 4 {{!}} 5 {{!}} 6 {{!}} 7 {{!}} 8}}
{{DEF|ISIF|0|for {{TAG|IBRION}}{{=}}0 (molecular dynamics)|2|else}}
{{DEF|ISIF|0|for {{TAGO|IBRION|0}} (molecular dynamics) or {{TAGO|LHFCALC|.TRUE.}}|2|else}}


Description: {{TAG|ISIF}} determines whether the stress tensor is calculated and which principal degrees-of-freedom are allowed to change in relaxation and molecular dynamics runs.
Description: Determines if the stress tensor is calculated and which ionic degrees of freedom are varied.
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{{TAG|ISIF}} determines whether the stress tensor is calculated.
For {{TAG|ISIF}}<math>\ge</math>2, the stress tensor is calculated. It is defined as the negative of the derivative of the energy
The calculation of the stress tensor is relatively time-consuming, and therefore by default switched off for ''ab initio'' molecular dynamics runs.
<math>  E </math> with respect to the strain tensor <math>\eta_{ji}</math>:
The forces are always calculated.


In addition {{TAG|ISIF}} determines which degrees-of-freedom (ionic positions, cell volume, and cell shape) are allowed to change.
:<math>  \sigma_{ij} = - \frac{\delta E} {\delta \eta_{ji}} </math>.


The following table shows all combinations:
This might be different from other first principles codes.
A positive in the diagonals means that the system is under compressive strain and wants to expand. A negative value implies that the system is under tensile strain and wants to reduce its volume. The stress tensor is symmetric <math>\sigma_{ij}=\sigma_{ji}</math>, and, thus, it has six independent entries.
The calculation of the stress tensor is relatively time-consuming, and, therefore, by default, it is switched off in some cases. The forces are always calculated.
 
{{TAG|ISIF}} also determines which degrees of freedom (ionic positions, cell volume, and cell shape) of the structure are allowed to change.


:{| cellpadding="5" cellspacing="0" border="1"
:{| cellpadding="5" cellspacing="0" border="1"
|{{TAG|ISIF}} ||colspan="2" style="text-align: center;"| calculate ||colspan="3" style="text-align: center;"| degrees-of-freedom  
|{{TAG|ISIF}} ||colspan="2" style="text-align: center;"| calculate ||colspan="3" style="text-align: center;"| degrees-of-freedom  
|-
|-
| || forces || Stress tensor || positions || cell shape || cell volume
| || forces || stress tensor || positions || cell shape || cell volume
|-
|-
| 0 || yes || no  || yes || no  || no   
| 0 || yes || no  || yes || no  || no   
Line 33: Line 36:
|-
|-
| 7 || yes || yes || no  || no  || yes
| 7 || yes || yes || no  || no  || yes
|-
| 8 || yes || yes || yes  || no  || yes
|}
|}


For {{TAG|ISIF}}=1, only the trace of the stress tensor is calculated. This means only the total pressure is correctly written to the {{FILE|OUTCAR}} file. See the line:
* For {{TAGO|ISIF|1}}, only the trace of the stress tensor is calculated. This means only the total pressure is correct and can be read off in the line:


  external pressure =      ... kB
  external pressure =      ... kB


The individual components of the stress tensor are not reliable in that case.
:The individual components of the stress tensor are not reliable in this case and must be disregarded.
* Accuracy
{{NB|warning|The PAW basis for the [[:Category:Electronic minimization|electronic minimization]] is not adjusted when the structure is varied during a calculation.|:}}
:Therefore, carefully consider effects such as [[Energy_vs_volume_Volume_relaxations_and Pulay_stress|Pulay stress]] and choose generous settings for the [[:Category:Electronic minimization|electronic minimization]]. Generally, volume changes should be done only with an increased energy cutoff, e.g., {{TAGO|ENCUT|1.3&times;max({{TAG|ENMAX}})}}, and {{TAGO|PREC|High}}.
* To further control the ionic degrees of freedom that can vary during the calculation, set `Selective dynamics` in the {{FILE|POSCAR}} file.
* {{TAGO|ISIF|8}} is only available as of VASP.6.4.1.
== Related tags and articles ==
{{TAG|IBRION}},
[[structure optimization]],
[[:Category:Ensembles| Ensembles]],
Selective-dynamics mode of the {{FILE|POSCAR}} file,
{{TAG|LATTICE_CONSTRAINTS}}


Before you perform relaxations in which the volume or the cell shape is allowed to change you must read and understand the section on [[Energy_vs_volume_Volume_relaxations_and Pulay_stress|energy vs. volume, volume relaxations, and Pulay stress]]. In general volume changes should be done only with an increased energy cutoff, ''i.e.'', {{TAG|ENCUT}}=1.3&times;max({{TAG|ENMAX}}) or {{TAG|PREC}}=High.
{{sc|ISIF|Examples|Examples that use this tag}}
 
== Related Tags and Sections ==
{{TAG|IBRION}},
{{TAG|ISIF}},
{{TAG|LEPSILON}},
{{TAG|LCALCEPS}}


----
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[[The_VASP_Manual|Contents]]


[[Category:INCAR]][[Category:Dynamics]]
[[Category:INCAR tag]][[Category:Ionic minimization]][[Category:Molecular dynamics]][[Category:Symmetry]]

Latest revision as of 11:49, 18 October 2024

ISIF = 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 

Default: ISIF = 0 for IBRION = 0 (molecular dynamics) or LHFCALC = .TRUE.
= 2 else

Description: Determines if the stress tensor is calculated and which ionic degrees of freedom are varied.


For ISIF2, the stress tensor is calculated. It is defined as the negative of the derivative of the energy with respect to the strain tensor :

.

This might be different from other first principles codes. A positive in the diagonals means that the system is under compressive strain and wants to expand. A negative value implies that the system is under tensile strain and wants to reduce its volume. The stress tensor is symmetric , and, thus, it has six independent entries. The calculation of the stress tensor is relatively time-consuming, and, therefore, by default, it is switched off in some cases. The forces are always calculated.

ISIF also determines which degrees of freedom (ionic positions, cell volume, and cell shape) of the structure are allowed to change.

ISIF calculate degrees-of-freedom
forces stress tensor positions cell shape cell volume
0 yes no yes no no
1 yes trace only yes no no
2 yes yes yes no no
3 yes yes yes yes yes
4 yes yes yes yes no
5 yes yes no yes no
6 yes yes no yes yes
7 yes yes no no yes
8 yes yes yes no yes
  • For ISIF = 1, only the trace of the stress tensor is calculated. This means only the total pressure is correct and can be read off in the line:
external pressure =      ... kB
The individual components of the stress tensor are not reliable in this case and must be disregarded.
  • Accuracy
Warning: The PAW basis for the electronic minimization is not adjusted when the structure is varied during a calculation.
Therefore, carefully consider effects such as Pulay stress and choose generous settings for the electronic minimization. Generally, volume changes should be done only with an increased energy cutoff, e.g., ENCUT = 1.3×max(ENMAX), and PREC = High.
  • To further control the ionic degrees of freedom that can vary during the calculation, set `Selective dynamics` in the POSCAR file.
  • ISIF = 8 is only available as of VASP.6.4.1.

Related tags and articles

IBRION, structure optimization, Ensembles, Selective-dynamics mode of the POSCAR file, LATTICE_CONSTRAINTS

Examples that use this tag