SYSTEM-S TRMX & SPGR


SYSTEM-S TOOLS PLATON HOMEPAGE

TRMX

The first step in the determination of the proper spacegroup concerns the determination of the associated Bravais lattice and Laue group symmetry.

A number of potentially appropriate lattices, consistent with the metrical symmetry, are shown. These were generated using the LEPAGE algorithm using a generous tolerance.

An averaging index is calculated for each lattice/laue group presented, along with an indication of completeness and redundancy.

Applicable lattices (with associated transformation matrices and Laue average indices) are listed.


SPGR (name) (criterium)

The second step in the determination of the proper space group involves the selection from a list of space groups compatible with the current Laue group and observed systematic extinctions.

In the ideal case, for extinction conditions, should be in the order of 0.0 to 1.0 for the "odd's" and large for the "even's" so that their ratio deviates significantly from 1.0 (last column).

In addition, the largest I/sigma and h,k,l are listed for the most 'non-fitting' reflection. Closer inspection of those reflections may be in order. (Causes include twinning, overlap with nearly reflections, Fe contamination and 1/2 wavelength overtones for CCD data).

There are several (empirical) thresholds to handle 'real-world' cases. One of which is that the T/F ratio should be above 5.0 to be considered an 'extinction'. This value can be changed when desired by specifying a criterium value (e.g. use the instruction SPGR 3.0 via the keyboard).

By default, space groups in standard settings are suggested (resulting in a non unit transformation matrix for the reflection data on the HKLF 4 shelx instruction).

S suggests a suitable space group based on several criteria, including the frequency in the CSD and the optional additional information that the compound involved is chiral.

An alternative space group from the list can be selected either by clicking on the appropriate line or explicitly from the keyboard. (E.g. SPGR Pc).

The non-standard space group P21/n can be selected (keyboard instruction SPGR P21/n) when automatic transformation to P21/c is suggested but P21/n considered more suitable for the current study (e.g to simplify the description of a twinning law).

Note: Either P21/c or P21/n can be choosen (i.e. not both).


23-Nov-1999 A.L.Spek