`ISYM`= `-1 | 0 | 1 | 2 | 3`

Default: | |

ISYM=1 |
if VASP runs with US-PP's |

=2 | if PAW data sets are used |

switch symmetry on (`ISYM`=1, 2 or 3) or off (`ISYM`=-1 or 0).
For `ISYM`=2 a more efficient, memory conserving symmetrisation
of the charge density is used. This reduces memory requirements
in particular for the parallel version.

For `ISYM`=3, the forces and the stress tensor only are symmetrized, whereas
the charge density is left unsymmetrized (VASP.5.1 only). This option might
be useful in special cases, where charge/orbital ordering lowers the crystal symmetry,
and the user wants to conserve the symmetry of the positions during relaxation.
However, the flag must be used with great caution, since a lower symmetry due to
charge/orbital ordering in principle also requires to sample the Brillouin zone
using a k-point mesh compatible with the lower symmetry caused by charge/orbital ordering.

The program determines automatically the point group symmetry and the space group
according to the POSCAR file and the line
`MAGMOM` in the INCAR file.
The `SYMPREC`-tag (VASP.4.4.4 and newer versions only)
determines how accurate the positions in the POSCAR
file must be. The default is 10, which is usually suffiently
large even if the POSCAR file has been generated with a single
precision program. Increasing the `SYMPREC` tag means, that the positions
in the POSCAR file can be less accurate.
During the symmetry analysis, VASP determines

- the Bravais lattice type of the supercell,
- the point group symmetry and the space group of the supercell with basis (static and dynamic) - and prints the names of the group (space group: only 'family'),
- the type of the generating elementary (primitive) cell if the supercell is a non-primitive cell,
- all 'trivial non-trivial' translations (= trivial translations of the generating elementary cell within the supercell) -- needed for symmetrisation of the charge,
- the symmetry-irreducible set of k-points if automatic k-mesh generation was used and additionally the symmetry-irreducible set of tetrahedra if the tetrahedron method was chosen together with the automatic k-mesh generation and of course also the corresponding weights ('symmetry degeneracy'),
- and tables marking and connecting symmetry equivalent ions.

- First the point group symmetry of the lattice (as supplied by the user) is determined.
- Then tests are performed, whether the basis breaks symmetry. Accordingly these symmetry operations are removed.
- The initial velocities are checked for symmetry breaking.
- Finally, it is checked wheter
`MAGMOM`breaks the symmetry. Correspondingly the magnetic symmetry group is determined (VASP.4.4.4 and newer releases only; if you use older version please also see section 6.13).

The program symmetrizes automatically:

- The total charge density according to the determined space group
- The forces on the ions according to the determined space group.
- The stress tensor according to the determined space group

*Why is symmetrisation necessary:*
Within LDA the symmetry of the supercell and the charge density
are always the same. This symmetry is broken, because
a symmetry-irreducible set of k-points is used for the calculation.
To restore the correct charge density and the correct forces it is
necessary to symmetrise these quantities.

It must be stressed that VASP does *not* determine the
symmetry elements of the primitive cell. If the supercell
has a lower symmetry than the primitive cell only the
lower symmetry of the supercell is used in the calculation.
In this case one should not expect that forces
that should be zero according to symmetry will be precisely
zero in actual calculations.
The symmetry of the primitive cell
is in fact broken in several places in VASP:

- local potential:
In reciprocal space, the potential should be zero, if G is not a reciprocal lattice vector of the primitive cell. For

`PREC`=Med, this is not guaranteed due to "aliasing" or wrap around and the charge density (and therefore the Hartree potential) might violate this point. But even for`PREC`=High, small errors are introduced, because the exchange correlation potential is calculated in real space. - k-points:
In most cases, the automatic k-point grid does not have the symmetry of the primitive cell.

N.B. Requests for support are to be addressed to: vasp.materialphysik@univie.ac.at