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ISIF= 0 | 1 | 2 | 3 | 4 | 5 | 6

=2 else

ISIF controls 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 MD's. Forces are always calculated.

In addition ISIF determines which degrees of freedom (ions, cell volume, cell shape) are allowed to change.

The following table shows the meaning of ISIF. At the moment cell changes are only supported for relaxations and nor fot molecular dynamics simulations.

ISIF calculate calculate relax change change
  force stress tensor ions 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
$ ^*$ Trace only means that only the total pressure, i.e. the line
external pressure =      ... kB
is correct. The individual components of the stress tensor are not reliable in that case. This switch must be used with caution. Mind: Before you perform relaxations in which the volume or the cell shape is allowed to change you must read and understand section 7.6. In general volume changes should be done only with a slightly increased energy cutoff (i.e. ENCUT=1.3 * default value , or PREC=High in VASP.4.4).

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