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Next: 7.25 LCORR-tag Up: 7 The INCAR File Previous: 7.23 IWAVPR-tag

7.24 ISYM-tag

displaymath4952

Default 1

switch symmetry stuff ON (1 or 2) or OFF (0). For ISYM=2 a more efficient memory conserving symmetrization 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 symmetry analyses is done in three 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 symmetrises 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 cases 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: 7.25 LCORR-tag Up: 7 The INCAR File Previous: 7.23 IWAVPR-tag

MASTER USER VASP
Mon Mar 29 10:38:29 MEST 1999