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Old way of Space group determination: Difference between revisions
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Old way of Space group determination (view source)
Revision as of 18:28, 25 November 2007
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** modify or insert UNIT_CELL_PARAMETERS= according to table, but make them obey space group requirements (e.g. orthorhombic: all angles 90°, tetragonal and trigonal: a=b, and so on). | ** modify or insert UNIT_CELL_PARAMETERS= according to table, but make them obey space group requirements (e.g. orthorhombic: all angles 90°, tetragonal and trigonal: a=b, and so on). | ||
** run XDS with JOB=CORRECT. Inspect [[CORRECT.LP]] and note R-factors, I/sigma and a and b modifiers of standard deviations | ** run XDS with JOB=CORRECT. Inspect [[CORRECT.LP]] and note R-factors, I/sigma and a and b modifiers of standard deviations | ||
** for many Bravais lattices, there are several possible point groups (tp and tI: 4, 422; hP: 3, 6, 312, 321, 622; hR: 3, 32; cP, cF and cI: 23 and 432) and thus several possible space groups. In these cases, | ** for many Bravais lattices, there are several possible point groups (tp and tI: 4, 422; hP: 3, 6, 312, 321, 622; hR: 3, 32; cP, cF and cI: 23 and 432) and thus several possible space groups. In these cases, each point group has to be tested: the SPACE_GROUP_NUMBER has to be changed to one of those consistent with the point group, and the CORRECT step re-run. | ||
* repeat for each possible lattice character | * repeat for each possible lattice character | ||
* finally decide on the correct Bravais lattice and point group by comparing R-factors (in particular R_meas), and, from there, come up with possible space groups by looking at the table of systematic absences along the h,0,0 0,k,0 and 0,0,l axes of the diffraction pattern. | * finally decide on the correct Bravais lattice and point group by comparing R-factors (in particular R_meas), and, from there, come up with possible space groups by looking at the table of systematic absences along the h,0,0 0,k,0 and 0,0,l axes of the diffraction pattern. | ||
Be aware of the possibility of [http://www.ccp4.ac.uk/dist/html/twinning.html twinning], and of the different, but equivalent ways to index a given diffraction pattern ( | Be aware of the possibility of [http://www.ccp4.ac.uk/dist/html/twinning.html twinning], and of the different, but equivalent ways to index a given diffraction pattern (reindexing)! | ||