ISa: Difference between revisions

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On the other hand, I have sometimes obtained values less than 10 with good test crystals, clearly indicating strong systematic errors. It is always good to discuss this with the people who are responsible for the beamline. They might know what is broken, or might be able to find out what went wrong.

Values of ISa below 5 are almost always a sign that something is severely wrong, and that the data are useless. Possible reasons are: wrong geometry specified in XDS.INP, wrong spacegroup (too high symmetry), high radiation damage, mechanical deficiencies during measurement.

'''Values of ISa below 5 are almost always a sign that something is severely wrong, and that the data are useless. Possible reasons are: wrong geometry specified in XDS.INP, wrong spacegroup (too high symmetry), high radiation damage, mechanical deficiencies during measurement.'''

A high systematic error may be partly compensated by high multiplicity. Conversely, high multiplicity is not needed to solve a structure, if the data have a high I/Sigma(I)<sup>asymptotic</sup>.

For molecular replacement and refinement, a high value of I/Sigma(I)<sup>asymptotic</sup> is not strictly needed (but the maps are better with better data, and the model R values lower!).

Please note that for electron diffraction, there is a systematic disagreement between symmetry-related reflections due to dynamic diffraction. Therefore one expects lower values of ISa for electron diffraction than for X-ray and neutron diffraction.

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 On the other hand, I have sometimes obtained values less than 10 with good test crystals, clearly indicating strong systematic errors. It is always good to discuss this with the people who are responsible for the beamline. They might know what is broken, or might be able to find out what went wrong.
-Values of ISa below 5 are almost always a sign that something is severely wrong, and that the data are useless. Possible reasons are: wrong geometry specified in XDS.INP, wrong spacegroup (too high symmetry), high radiation damage, mechanical deficiencies during measurement.
+'''Values of ISa below 5 are almost always a sign that something is severely wrong, and that the data are useless. Possible reasons are: wrong geometry specified in XDS.INP, wrong spacegroup (too high symmetry), high radiation damage, mechanical deficiencies during measurement.'''
 A high systematic error may be partly compensated by high multiplicity. Conversely, high multiplicity is not needed to solve a structure, if the data have a high I/Sigma(I)<sup>asymptotic</sup>.
 For molecular replacement and refinement, a high value of I/Sigma(I)<sup>asymptotic</sup> is not strictly needed (but the maps are better with better data, and the model R values lower!).
+Please note that for electron diffraction, there is a systematic disagreement between symmetry-related reflections due to dynamic diffraction. Therefore one expects lower values of ISa for electron diffraction than for X-ray and neutron diffraction.