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 $\ge$ 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 $\eta_{ji}$ :

\sigma_{ij} = - \frac{\delta E} {\delta \eta_{ji}}

.

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 $\sigma_{ij}=\sigma_{ji}$ , 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.

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