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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.

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.

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. In general volume changes should be done only with an increased energy cutoff, i.e., ENCUT=1.3×max(ENMAX) or PREC=High.

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