Space group determination
In case of a crystal with an unknown space group (SPACE_GROUP_NUMBER=0 in XDS.INP), XDS (since version June 2008) helps the user in determination of the correct space group, by suggesting possible space groups compatible with the lattice symmetry of the data, and by calculating the Rmeas for these space groups.
Space group selected by user
The resulting list (in CORRECT.LP) should give the user enough information to pick the correct space group herself. She may then put suitable lines with SPACE_GROUP_NUMBER=, UNIT_CELL_CONSTANTS= into XDS.INP and re-run the CORRECT step to obtain the desired result. (The REIDX= line is no longer required; XDS figures the matrix out.)
Space group selected by XDS
Additionally, XDS (or rather, the CORRECT step) makes an attempt to pick the correct space group automatically: it chooses that space group which has the highest symmetry and still a tolerable Rmeas compared to the Rmeas the data have in P1.
Parameters influencing the automatic decision
What is the meaning of "tolerable" in this context? The automatic choice is influenced by a number of decision constants that may be put into XDS.INP but which have defaults as indicated below:
- MAX_CELL_AXIS_ERROR= 0.03 ! relative deviation of unconstrained cell axes from those constrained by lattice symmetry
- MAX_CELL_ANGLE_ERROR= 0.3 ! degrees deviation of unconstrained cell angles from those constrained by lattice symmetry
- TEST_RESOLUTION_RANGE= 10.0 5.0 ! resolution range for calculation of Rmeas
- MIN_RFL_Rmeas= 50 ! at least this number of reflections are required
- MAX_FAC_Rmeas= 2.0 ! factor to multiply P1-Rmeas with to still be acceptable
The user may experiment with adjusting these values to make the automatic mode of space group determination more successful. However, it is much easier to just input the space group and cell parameters that the user thinks are correct.
Notes
There is still the page Old way of Space group determination