The SYM_DELTA tag

Warning

SYM_DELTA is not well tested yet, and has at least one known bug. Use of SYMMETRY_FIX instead of SYM_DELTA is encouraged where possible.

SYM_DELTA is a tag in the DISPLACEMENTS file that allows you to temporarily change the symmetry of your slab, such that displacements then assigned to atoms will be applied to fewer or no “symmetry-equivalent” atoms. If you require lowering of symmetry, this will apply only to those input lines between two SYM_DELTA lines (e.g., start a block with one line turning off symmetry or changing the symmetry group, and end it with one line turning the full symmetry back on or changing the symmetry to another group).

Example

= GEO_DELTA
O 1 z = -0.05 0.05 0.005      ! Oxygen atom 1 (and symmetry-equivalent atoms) will be displaced in z direction over the range [-0.05, 0.05] with step 0.005
SYM_DELTA = False
Ir 1 3-5 z = -0.05 0.05 0.01  ! Iridium atoms 1 and 3-5 (but NOT their symmetry-equivalent atoms) will be displaced in z direction over the range [-0.05, 0.05] with step 0.01
SYM_DELTA = True

= VIB_DELTA
Ir 1-6 = -0.05 0.05 0.02      ! Vibrational amplitude of iridium atoms 1-6 (and symmetry-equivalent atoms) will be varied over the range [-0.05, 0.05] with step 0.02

Acceptable values

  • T, True, F, False (not case sensitive): False turns off symmetry linking entirely, which is equivalent to setting the symmetry group to p1. True turns symmetry linking back on.

  • One can also directly specify a symmetry group with SYM_DELTA = group. That group will then be used to restrict geometrical displacements or link symmetry-equivalent atoms. This is functionally equivalent to changing the value of SYMMETRY_FIX, but only for the operations that follow. Note that only symmetry reduction from the overall slab symmetry is allowed. See SYMMETRY_FIX for a more detailed explanation of allowed symmetry changes.

The use of SYM_DELTA should be reserved for highly specific cases and is generally discouraged. If the desired effect can be achieved by lowering the overall symmetry, then using the SYMMETRY_FIX tag of the PARAMETERS file is always preferable.