ISIF: Difference between revisions

Revision as of 10:51, 3 October 2023

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 ISIF $\geq$ 2, the stress tensor is calculated. It is defined as the negative of the derivative of the energy $E$ with respect to the strain tensor $\epsilon _{ij}$ :

\sigma _{ij}=-{\frac {\delta E}{\delta \epsilon _{ij}}}

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 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 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. 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.

@@ Line 2: / Line 2: @@
 {{DEF|ISIF|0|for {{TAG|IBRION}}{{=}}0 (molecular dynamics) or {{TAG|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.
-{{NB|mind|ISIF{{=}}8 is only available as of VASP.6.4.1.}}
 ----
-{{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>
-The forces are always calculated.
+with respect to the strain tensor <math>   \epsilon_{ij} </math>:
+:<math>  \sigma_{ij} = - \frac{\delta E} {\delta \epsilon_{ij}} </math>
-In addition {{TAG|ISIF}} determines which degrees-of-freedom (ionic positions, cell volume, and cell shape) are allowed to change.
+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 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.
-The following table shows all combinations:
+{{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"
 |{{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
@@ Line 37: / Line 40: @@
 | 8 || yes || yes || yes  || no  || yes
 |}
-In VASP the stress tensor is defined as the negative of the derivative of the energy
-<math>  E </math>
-with respect to the strain tensor:
-<math>   \epsilon_{ij} </math>
-<math>  \sigma_{ij} = - \frac{\delta E} {\delta \epsilon_{ij}} </math>
+* For {{TAG|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:
-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.
-For {{TAG|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.
+: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.|:}}
-{{NB|mind|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.}}
+: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., {{TAG|ENCUT}}{{=}}1.3&times;max({{TAG|ENMAX}}), and {{TAG|PREC}}{{=}}High.
-{{NB|mind| When {{TAG|LHFCALC}}{{=}}.TRUE (for Hartree-Fock/hybrid calculations), by default {{TAG|ISIF}}{{=}}0 and the stress tensor is not calculated.}}
+* To further control the ionic degrees of freedom that can vary during the calculation, set `Selective dynamics` in the {{FILE|POSCAR}} file.
+* ISIF{{=}}8 is only available as of VASP.6.4.1.
 == Related tags and articles ==
 {{TAG|IBRION}},
-{{TAG|LEPSILON}},
+[[:Category:Ensembles| Ensembles]],
-{{TAG|LCALCEPS}}
+Selective-dynamics mode of the {{FILE|POSCAR}} file
 {{sc|ISIF|Examples|Examples that use this tag}}