Custom Symmetry
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Dialog Box: Custom Symmetry [ Symmetry Dialog ]

CRYSCON can access all the standard orientations for space groups in the International Tables but there are still a few possibilities for alternate settings of symmetry elements with respect to structure axes which are not listed in the Tables . In some cases displacement, rotation or permutation of atomic coordinates may not be convenient, and it may be easier to enter the symmetry operations for the non-standard setting. These are sometimes given in reports on structures (e.g. Wyckoff, Crystal Structures ). Note however that often only the operations for special positions are given: CRYSCON needs all the operations for the general position.

Some Import formats do not give a space-group symbol, and symmetry is supplied in the form of operations as xyz, or as actual numerical matrices. CRYSCON converts numerical matrices to xyz form.

The Symbol is not used by CRYSCON for transformations, unless you switch to the Space Group Symmetry option. However, the Powder and Precession options require a valid space-group symbol - they must use the symmetry from the symbol rather than the custom symmetry.
Get Space Group Symbol. This button will attempt to identify the space group from the operations, the lattice type and the centric/acentric setting. If the operations contain an inversion center or lattice centerings, CRYSCON should identify them and they can be removed at your option. Clicking the Change Symmetry Option button will automatically causes an attempt to identify the space group.

If you specify Centric, rather than Acentric, you must enter only half of the symmetry operations (as given in the International Tables ), related to any others by the center. The two sets or halves of the operations related by the center will have opposite signs on the x, y and z coordinates. The Centric choice implies that the origin of coordinates is on a center of inversion. If the space group is actually centric, but the origin is not on a center, select Acentric and enter all the operations. Note that for a transformation the atoms can be shifted (Pre-symmetry translations) in the Tranformation Parameters dialog, to place the origin either on or off an inversion center.

The Bravais lattice type is in most cases the capital letter which begins the space-group symbol. However, CRYSCON uses a somewhat different convention from the standard for trigonal crystals: if you are using hexagonal axes for a rhombohedral crystal (whose standard space group symbol will always begin with "R"), enter "R" - if you are using the primitive rhombohedral axes, enter "P".

In the Operations group, you must enter all the "symmetry operations" for the most general position in the structure. Click on the Add Line button to add a line, Delete to remove one. Note that there should be only one operation per line. Enter or edit the line in the edit box to the right of the list.