ISIF

ISIF = 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 

Default: ISIF	= 0	for IBRION=0 (molecular dynamics)
	= 2	else

Description: ISIF determines whether the stress tensor is calculated and which principal degrees-of-freedom are allowed to change in relaxation and molecular dynamics runs.

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ISIF determines whether the stress tensor is calculated. The calculation of the stress tensor is relatively time-consuming, and therefore by default switched off for ab initio molecular dynamics runs. The forces are always calculated.

In addition ISIF determines which degrees-of-freedom (ionic positions, cell volume, and cell shape) are allowed to change.

The following table shows all combinations:

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

For ISIF=1, only the trace of the stress tensor is calculated. This means only the total pressure is correctly written to the OUTCAR file. See the line:

external pressure =      ... kB

The individual components of the stress tensor are not reliable in that case.

Related Tags and Sections

IBRION, ISIF, LEPSILON, LCALCEPS

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