ISIF

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[math]\displaystyle{ \ge }[/math]2, the stress tensor is calculated. It is defined as the negative of the derivative of the energy [math]\displaystyle{ E }[/math] with respect to the strain tensor [math]\displaystyle{ \eta_{ji} }[/math]:

[math]\displaystyle{ \sigma_{ij} = - \frac{\delta E} {\delta \eta_{ji}} }[/math].

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]\displaystyle{ \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.

Tip: You can get information about the stress at each ionic step using NWRITE = 0,1,2,3.

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.

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