SYMMETRY_CELL_TRANSFORM

SYMMETRY_CELL_TRANSFORM defines the base unit cell on which symmetry operations should be found and used for linking atom movements. All other atoms are assumed to be translationally symmetric, and will be linked to the base cell accordingly. The relationship between the symmetry base cell and the POSCAR unit cell is defined by SYMMETRY_CELL_TRANSFORM in the same way as the relationship between the bulk unit cell and the POSCAR unit cell is defined by SUPERLATTICE.

Note

SYMMETRY_CELL_TRANSFORM may be updated automatically during initialization if a reducible supercell is detected.

Default: (1x1)

Syntax examples: see SUPERLATTICE page

SYMMETRY_CELL_TRANSFORM is relevant in the context of domain calculations, where the calculations for the structures in the different domains must all be run with the same supercell, even when their own unit cell is smaller. For example, when fitting a mixture of a (1x1) and a (2x1) structure, the (1x1) structure has to be re-calculated on a (2x1) unit cell. However, during this calculation, it makes sense to still base the symmetry search for the (1x1) on the base (1x1) unit cell instead of the (2x1), which may lower symmetry. In addition, atoms that are completely equivalent on the (1x1) should be linked by translational symmetry on the (2x1) supercell. SYMMETRY_CELL_TRANSFORM performs this function, essentially defining a ‘real’ surface unit cell which should be used as a basis for atom linking (which may differ from the bulk and from the POSCAR unit cell). Note that this effectively makes the supercell calculation identical to a calculation on the smaller cell, i.e. the resulting supercell structure is reducible to the base cell defined by SYMMETRY_CELL_TRANSFORM without any conflicts or loss of information. Therefore, outside of the context of domain calculations, it is usually better to use a smaller POSCAR unit cell rather than the SYMMETRY_CELL_TRANSFORM parameter, because the results will be the same, but calculating on a larger cell is much less computationally efficient.

Note that the base unit cell defined by SYMMETRY_CELL_TRANSFORM may still be automatically reshaped to a higher-symmetry / lower-circumference form, in the same way as during optimization of the POSCAR unit cell.

TODO Florian (@Michele, Alex: Florian said this is on his TODO list): add figures to help