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Space group determination: Difference between revisions
→Space groups to consider based on CORRECT suggestion: exchange the first two columns
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In many cases the automatic choice is the correct one, and re-running the [[CORRECT.LP|CORRECT]] step is then not necessary. However, neither the correct enantiomorph nor screw axes (see below) are determined automatically by XDS. | In many cases the automatic choice is the correct one, and re-running the [[CORRECT.LP|CORRECT]] step is then not necessary. However, neither the correct enantiomorph nor screw axes (see below) are determined automatically by XDS. | ||
=== Space groups | === Space groups as combinations of symmetry elements === | ||
Space group determination entails the following steps: | |||
# determine the '''Laue group''': this is the symmetry of the intensity-weighted point lattice (diffraction pattern). 1,2,3,4,6=n-fold rotation axis; -n means inversion centre; m means mirror. | |||
# find out about '''Bravais type''': the second letter signifies centering (P=primitive; C=centered on C-face; F=centered on all faces; I=body-centered; R=rhombohedral). The first letter (a=triclinic; m=monoclinic; o=orthorhombic; h=trigonal or hexagonal; c=cubic) is redundant since it can be inferred from the Laue group. | |||
# possible spacegroups are now given in '''columns 3 and 4'''; CORRECT always suggests the one given in column 3 but this is no more likely than those in column 4. | |||
# choose according to screw axis; this may result in two possibilities (enantiomorphs). | |||
# determine correct enantiomorph - this usually means that one tries to solve the structure in both spacegroups, and only one gives a sensible result (like helices that are right-handed, amino acids of the L type). | |||
{| cellpadding="10" cellspacing="0" border="1" | {| cellpadding="10" cellspacing="0" border="1" | ||
! Laue group | |||
! Bravais type | ! Bravais type | ||
! | ! CORRECT suggestion | ||
! other possibilities | |||
! | |||
! Comment | ! Comment | ||
|- | |- | ||
| | | -1 || aP || 1 || - || | ||
|- | |- | ||
| | | 2m || mP || 3 || 4 || screw axis extinctions let you decide | ||
|- | |- | ||
| | | 2m || mC || 5 || - || The [[pointless]] article discusses why I2 (mI) should not be used. | ||
|- | |- | ||
| | | mmm || oP || 16 || 17, 18, 19 || screw axis extinctions let you decide. The [[pointless]] article discusses why CCP4's 1017 (P2<sub>1</sub>22), 2017 (P22<sub>1</sub>2), 2018 (P2<sub>1</sub>22<sub>1</sub>) , 3018 (P22<sub>1</sub>2<sub>1</sub>) are not needed and should not be used. | ||
|- | |- | ||
| | | mmm || oC || 21 || 20 || screw axis extinctions let you decide | ||
|- | |- | ||
| | | mmm || oF || 22 || - || | ||
|- | |- | ||
| | | mmm || oI || 23 || 24 || screw axis extinctions do '''not''' let you decide because the I centering results in h+k+l=2n and the screw axis extinction 00l=2n is just a special case of that. | ||
|- | |- | ||
| 4/m || tP || 75 || 76, 77, 78 || screw axis extinctions let you decide, except between 76/78 enantiomorphs | |||
|- | |- | ||
| | | 4/m || tI || 79 || 80 || screw axis extinctions let you decide | ||
|- | |- | ||
| | | 4/mmm || tP || 89 || 90, 91, 92, 93, 94, 95, 96 || screw axis extinctions let you decide, except between 91/95 and 92/96 enantiomorphs | ||
|- | |- | ||
| 4/mmm || tI || 97 || 98 || screw axis extinctions let you decide | |||
|- | |- | ||
| | | -3 || hP || 143 || 144, 145 || screw axis extinctions let you decide, except between 144/145 enantiomorphs | ||
|- | |- | ||
| | | -3 || hR || 146 || - || | ||
|- | |- | ||
| -3/m || hP || 149 || 151, 153 || screw axis extinctions let you decide, except between 151/153 enantiomorphs. Note: the twofold goes along the diagonal between a and b. | |||
|- | |- | ||
| -3/m || hP || 150 || 152, 154 || screw axis extinctions let you decide, except between 152/154 enantiomorphs. Note: compared to previous line, the twofold goes along a. | |||
|- | |- | ||
| -3/m || hR || 155 || - || | |||
|- | |- | ||
| 6/m || hP || 168 || 169, 170, 171, 172, 173 || screw axis extinctions let you decide, except between 169/170 and 171/172 enantiomorphs | |||
|- | |- | ||
| 6/mmm || hP || 177 || 178, 179, 180, 181, 182 || screw axis extinctions let you decide, except between 178/179 and 180/181 enantiomorphs | |||
|- | |- | ||
| m-3 || cP || 195 || 198 || screw axis extinctions let you decide | |||
|- | |- | ||
| m-3 || cF || 196 || - || | |||
|- | |- | ||
| m-3 || cI || 197 || 199 || screw axis extinctions do '''not''' let you decide because the I centering results in h+k+l=2n and the screw axis extinction 00l=2n is just a special case of that. | |||
|- | |- | ||
| m-3m || cP || 207 || 208, 212, 213 || screw axis extinctions let you decide, except between 212/213 enantiomorphs | |||
|- | |- | ||
| m-3m || cF || 209 || 210 || screw axis extinctions let you decide | |||
|- | |- | ||
| m-3m || cI || 211 || 214 || screw axis extinctions let you decide | |||
|- | |- | ||
|} | |} | ||
If you find an error in the table please send an email to kay dot diederichs at uni-konstanz | |||
If you find an error in the table please send an email to kay dot diederichs at uni-konstanz dot de ! | |||
== Space group selected by user == | == Space group selected by user == | ||