ISYM= -1 | 0 | 1 | 2 | 3
|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 program symmetrizes automatically:
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:
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.
In most cases, the automatic k-point grid does not have the symmetry of the primitive cell.