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 R_meas 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.

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 R_meas compared to the R_meas 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 R_meas
• MIN_RFL_Rmeas= 50  ! at least this number of reflections are required
• MAX_FAC_Rmeas= 2.0  ! factor to multiply P1-R_meas 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