== Space group selected by XDS: ambiguous with respect to enantiomorph and screw axes ==
In case of a crystal with an unknown space group (SPACE_GROUP_NUMBER=0 in [[XDS.INP]]), XDS (since [http://
www.mpimf-heidelberg.mpg.de /~kabsch/xds/html_doc/Release_Notes.html version June 2008]) helps the user in determination of the correct space group, by suggesting possible space groups compatible with the Laue symmetry and Bravais type of the data, and by calculating the R<sub>meas</sub> for these space groups.
XDS (or rather, the [[CORRECT.LP|CORRECT]] step) makes an attempt to pick the correct space group automatically: it chooses that space group which has the highest symmetry (thus yielding the lowest number of unique reflections) and still a tolerable R<sub>meas</sub> compared to the R<sub>meas</sub> the data have in any space group (which is most likely a low-symmetry space group - often P1).
There are two ways to have XDS choose an indexing consistent with some other dataset:
* using REFERENCE_DATA_SET= ! see also [[REFERENCE_DATA_SET]]
* using [http://
www.mpimf-heidelberg.mpg.de /~kabsch/xds/html_doc/xds_parameters.html#UNIT_CELL_A-AXIS= UNIT_CELL_A-AXIS=], UNIT_CELL_B-AXIS=, UNIT_CELL_C-AXIS= from a previous data collection run with the same crystal
One can also manually force a specific indexing, using the REIDX= keyword, but this is error-prone.