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Space group determination: Difference between revisions
→Space groups as combinations of symmetry elements
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Space group determination entails the following steps: | Space group determination entails the following steps: | ||
# determine the '''Laue | # determine the '''Laue class''': 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 (normally the - is written over the n); 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. | # 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. | # 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). | # choose according to screw axis (according to the table "REFLECTIONS OF TYPE H,0,0 0,K,0 0,0,L OR EXPECTED TO BE ABSENT (*)" in [[CORRECT.LP]]); 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). | # 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). | ||
== Table of space groups by Laue class and Bravais type == | |||
{| cellpadding="10" cellspacing="0" border="1" | {| cellpadding="10" cellspacing="0" border="1" | ||
! Laue class | ! Laue class | ||