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 Bravais lattice type of the supercell.
- The symmetry point and space group of the supercell with basis (static and dynamically!!) - 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').
- 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.
- Finally, the velocities are checked for symmetry breaking.
- MIND: Currently vasp does not perform a check whether the initial magnetic moments break the symmetry. This means that for anti-ferromagnetic calculations symmetry must be broken by ``hand''.

The program symmetrises 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

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:

- 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 Hatree 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.

Mon Mar 29 10:38:29 MEST 1999