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Space group determination: Difference between revisions
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* [[pointless]] helps with identification of screw axes, and its identification of Laue group is more sensitive than that implemented in CORRECT. However it is not fail-safe, and cannot tell the correct enantiomorph. | * [[pointless]] helps with identification of screw axes, and its identification of Laue group is more sensitive than that implemented in CORRECT. However it is not fail-safe, and cannot tell the correct enantiomorph. | ||
* only when the structure is satisfactorily refined can the chosen space group be considered established and correct. Until then, it is just a hypothesis and its alternatives (see the table) should be kept in mind. | * only when the structure is satisfactorily refined can the chosen space group be considered established and correct. Until then, it is just a hypothesis and its alternatives (see the table) should be kept in mind. | ||
== An example == | |||
The following example shows the difficulty in deciding between higher and lower symmetry. | |||
IDXREF reports: | |||
<pre> | |||
LATTICE- BRAVAIS- QUALITY UNIT CELL CONSTANTS (ANGSTROEM & DEGREES) | |||
CHARACTER LATTICE OF FIT a b c alpha beta gamma | |||
* 44 aP 0.0 74.0 78.6 124.0 108.2 105.1 90.4 | |||
* 31 aP 3.5 74.0 78.6 124.0 71.8 74.9 90.4 | |||
* 41 mC 7.4 235.7 78.6 74.0 90.4 106.1 89.7 | |||
* 37 mC 34.8 239.6 74.0 78.6 90.4 109.0 87.8 | |||
* 42 oI 38.7 74.0 78.6 226.6 90.2 92.2 90.4 | |||
14 mC 122.5 107.6 108.3 124.0 92.8 114.1 86.6 | |||
... | |||
</pre> | |||
so oI (space groups 23=I222 or 24=I2<sub>1</sub>2<sub>1</sub>2<sub>1</sub>) or the two mC (space group 5=C2) are possibilities for indexing, and of course P1. Integration in P1 (because of SPACE_GROUP_NUMBER=0 in XDS.INP) results in the following table for space group determination in CORRECT: | |||
<pre> | |||
SPACE-GROUP UNIT CELL CONSTANTS UNIQUE Rmeas COMPARED LATTICE- | |||
NUMBER a b c alpha beta gamma CHARACTER | |||
1 74.1 78.6 124.0 71.8 74.8 90.4 6959 22.5 5309 31 aP | |||
* 23 74.1 78.6 226.6 90.0 90.0 90.0 2735 39.5 9533 42 oI | |||
5 235.7 78.6 74.1 90.0 106.1 90.0 3922 31.0 8346 41 mC | |||
5 239.6 74.1 78.6 90.0 109.0 90.0 5111 30.7 7157 37 mC | |||
1 74.1 78.6 124.0 108.2 105.2 90.4 6959 22.5 5309 44 aP | |||
</pre> | |||
The automatic choice gave 23 as a result. This is because the associated R<sub>meas</sub> (39.5) is lower than MAX_FAC_RMEAS * (lowest R<sub>meas</sub>) = 2 * 22.5 = 45.0. | |||
However, the C2 values of 30.7 and 31.0 are significantly lower still, and the question needs further investigation. <code>[[pointless]] XDS_ASCII.HKL</code> gives: | |||