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Space group determination: Difference between revisions
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== Space group selected by XDS == | == Space group selected by XDS == | ||
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<sub>meas</sub> compared to the R<sub>meas</sub> the data have in | 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<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). | ||
In many cases the automatic choice may be the correct one, and re-running the CORRECT step is then not necessary. | In many cases the automatic choice may be the correct one, and re-running the CORRECT step is then not necessary. | ||
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== Space group selected by user == | == Space group selected by user == | ||
Even in case the space group selected by XDS should be incorrect, the resulting list (in CORRECT.LP) should give the user enough information to pick the correct space group herself. | Even in case the space group selected by XDS should be incorrect, the resulting list (in CORRECT.LP) should give the user enough information to pick the correct space group herself. The user 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.) | ||
== Influencing the selection by XDS == | == Influencing the selection by XDS == | ||
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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: | 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_AXIS_ERROR= 0.03 ! relative deviation of unconstrained cell axes from those constrained by lattice symmetry | ||
* MAX_CELL_ANGLE_ERROR= 0 | * MAX_CELL_ANGLE_ERROR= 3.0 ! 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<sub>meas</sub> | * TEST_RESOLUTION_RANGE= 10.0 5.0 ! resolution range for calculation of R<sub>meas</sub> | ||
* MIN_RFL_Rmeas= 50 ! at least this number of reflections are required | * MIN_RFL_Rmeas= 50 ! at least this number of reflections are required | ||
* MAX_FAC_Rmeas= 2.0 ! factor to multiply the R<sub>meas</sub> | * MAX_FAC_Rmeas= 2.0 ! factor to multiply the lowest R<sub>meas</sub> 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. | 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. | ||
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* There is still the old Wiki page [[Old way of Space group determination]] | * There is still the old Wiki page [[Old way of Space group determination]] | ||
* To prevent XDS from trying to find a better space group than P1, you could just use MAX_FAC_Rmeas= 1.0 | * To prevent XDS from trying to find a better space group than the one with the lowest R<sub>meas</sub> (often P1), you could just use MAX_FAC_Rmeas= 1.0 | ||