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Space group determination: Difference between revisions
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→Screw axes: make more explicit
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# a two-fold screw axis along an axis in reciprocal space (theoretically) results in zero intensity for the odd-numbered (e.g. 0,K,0 with K = 2*n + 1) reflections, leaving the reflections of type 2*n as candidates for medium to strong reflections (they don't ''have'' to be strong, but they ''may'' be strong!). | # a two-fold screw axis along an axis in reciprocal space (theoretically) results in zero intensity for the odd-numbered (e.g. 0,K,0 with K = 2*n + 1) reflections, leaving the reflections of type 2*n as candidates for medium to strong reflections (they don't ''have'' to be strong, but they ''may'' be strong!). | ||
# similarly, a three-fold screw axis (theoretically) results in zero intensity for the reflections of type 3*n+1 and 3*n+2, and allows possibly strong 3*n reflections . 3<sub>1</sub> and 3<sub>2</sub> cannot be distinguished - they are enantiomorphs. | # similarly, a three-fold screw axis (theoretically) results in zero intensity for the reflections of type 3*n+1 and 3*n+2, and allows possibly strong 3*n reflections . 3<sub>1</sub> and 3<sub>2</sub> cannot be distinguished - they are enantiomorphs. | ||
# analogously for four-fold and six-fold axes. | # analogously for four-fold screw axes: for H=0, K=0 reflections, 4<sub>1</sub> and 4<sub>3</sub> screws yield the rule L=4*n, and 4<sub>2</sub> yields the rule L=2*n . | ||
# analogously for six-fold screw axes: for H=0, K=0 reflections, 6<sub>1</sub> and 6<sub>5</sub> screws yield the rule L=6*n, 6<sub>2</sub> and 6<sub>4</sub> yield the rule L=3*n, and 6<sub>3</sub> yields the rule L=2*n . | |||
Once screw axes have been deduced from the patterns of intensities along H,0,0 0,K,0 0,0,L , the resulting space group should be identified in the lists of space group numbers printed in IDXREF.LP and CORRECT.LP, and the CORRECT step can be re-run. Those reflections that should theoretically have zero intensity are then marked with a "*" in the table. In practice, they should be (hopefully quite) weak, or even negative. | Once screw axes have been deduced from the patterns of intensities along H,0,0 0,K,0 0,0,L , the resulting space group should be identified in the lists of space group numbers printed in IDXREF.LP and CORRECT.LP, and the CORRECT step can be re-run. Those reflections that should theoretically have zero intensity are then marked with a "*" in the table. In practice, they should be (hopefully quite) weak, or even negative. | ||