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CORRECT.LP: Difference between revisions
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== Space group determination == | |||
The approach to space group determination is well explained in CORRECT.LP : | |||
XDS adopts the following approach. | |||
(1) it looks for possible symmetries of the crystal lattice | |||
(2) it computes a redundancy independent R-factor for all enantiomorphous | |||
point groups compatible with the observed lattice symmetry. | |||
(3) it selects the group which explains the intensity data at an acceptable, | |||
redundancy-independent R-factor (Rmeas, Rrim) using a minimum number of | |||
unique reflections. | |||
This approach does not test for the presence of screw axes. Consequently, | |||
orthorhombic cell axes will be specified in increasing length (following | |||
conventions), despite the possibility that different assignments for the | |||
cell axes could become necessary for space groups P222(1) and P2(1)2(1)2 | |||
containing one or two screw axes, respectively. | |||
The user can always override the automatic decisions by specifying the | |||
correct space group number and unit cell constants in XDS.INP and repeating | |||
the CORRECT step of XDS. This provides a simple way to rename orthorhombic | |||
cell constants if screw axes are present. | |||
In addition, the user has the option to specify in XDS.INP | |||
(a) a reference data set or | |||
(b) a reindexing transformation or | |||
(c) the three basis vectors (if known from processing a previous data set | |||
taken at the same crystal orientation in a multi-wavelength experiment). | |||
These features of XDS are useful for resolving the issue of alternative | |||
settings of polar or rhombohedral cells (like P4, P6, R3). | |||
Please note the sentence: '''This approach does not test for the presence of screw axes.''' The information about those reflections that may indicate screw axes is actually given in the table '''"REFLECTIONS OF TYPE H,0,0 0,K,0 0,0,L OR EXPECTED TO BE ABSENT (*)"''' (near the end of CORRECT.LP) but there is no automatic evaluation of that table that would result in screw axis assignment. | |||
Therefore, the '''space group determination of XDS only results in evaluation of the possible point groups that are compatible with the lattice symmetry'''. Space group determination of XDS only suggests one representative for each point group - the other possible space groups belonging to its point group are possibilities as well, but are not listed! For example, if XDS suggest space group 89 (P422), then any other space group, like 90, 91, 92, 93, 94, 95 and 96 is equally possible. | |||
If the user wants more automatic determination, it is suggested to run | |||
pointless XDS_ASCII.HKL | |||
== Scaling information == | |||
=== Details about the error model === | |||
== Statistics of reflections == | == Statistics of reflections == | ||