Space group determination by EXPO2011

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Space group determination by EXPO

       The different steps of the algorithm are here schematized.
1) The full pattern decomposition is performed, according to the Le Bail algorithm, in the space group having the largest Laue symmetry and no extinction conditions (e.g., P 1 2/m 1 in the monoclinic system, P 2/m 2/m 2/m for the orthorhombic, P 4/m m m for the tetragonal, P 6/m m m for the trigonal-hexagonal systems, Pm-3m for the cubic).
2) The extracted intensities are then normalized, according to the classical Wilson method, by using as space group the Laue group mentioned at the point 1).
3) The z statistics is used to define the probability of the extinction symbol.
4) For each crystal system the probabilities of the various extinction symbols are calculated and presented to the user in order of probability.
Use the button on the toolbar to carry out to next steps.



The window contains the extinction symbol ranked according to their calculated probability; when the user clicks on one of them, the list of compatible space groups is supplied. The frequency of the selected space group in the Cambridge Structural Database (CSD) is reported in terms of number of times it appears in the CSD and percentage. This information can be useful when different space groups are associated to an extinction group. Press the 'OK' button to accept the selected space group and continue with the structure solution.
Press the button  to access to a list of reflections that, according to the selected extinction symbol, should be systematically absent. For each reflection the following specifications are given: the order number of reflection, the Miller index, the symmetry operator responsible for the extinction and the corresponding extinction condition, the probability for the extinction, the type of reflection (single or overlapped).
In default the reflections appear ordered according to the extinction condition but you can change the order selecting a different order by click on the title column. The selected reflection on a list will be marked on the pattern.

The reflections that are expected to be absent for the selected extinction symbol are marked by a vertical bar. The bars are situated below the observed pattern, just above the set of bars defining the reflection positions, and have different colors depending on whether the reflection is single (or belongs to a whole extinct cluster) or in overlapping. By default, a black vertical bar marks single systematically absent reflections, while the rest of the absent reflections (those in overlapping) are marked with  a purple bar. This graphical support helps the user to discard unreliable extinction symbols even if estimated with large probability.


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