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ISYM-tag and SYMPREC-tag


\begin{displaymath}
ISYM = 0 \vert 1 \vert 2
\end{displaymath}

Default 1


switch symmetry stuff ON (1 or 2) or OFF (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. ISYM=2 is the default if PAW data sets are used. ISYM=1 is the default if VASP runs with US-PP's.

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$^{-5}$, 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 symmetry analyses is done in four steps:

The program symmetrises automatically:

Why is a 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:


next up previous contents
Next: LCORR-tag Up: The INCAR File Previous: IWAVPR-tag   Contents
Georg Kresse
2007-03-01