[phenixbb] Discrepancy between R-factors from phenix.refine vs phenix Generate "Table 1"
Edward A. Berry
BerryE at upstate.edu
Sat Jun 1 08:34:33 PDT 2013
Comparing below, i'm confused about the ability of the "L" test to distinguish twinning from underestimated symmetry.
And is the Wilson Ratio test an intensity moments test?
On 1 Jun 2013, at 02:15, "Terwilliger, Thomas C" <terwilliger at lanl.gov> wrote:
> I think it is important to remember that perhaps the best statistic we have right now for identification of whether a structure is twinned is the Wilson ratio <I**2> / <I>**2. For acentric reflections this is 2.0 for untwinned and 1.5 for perfect twin. This can of course be complicated by pseudo-centering. Aside from that issue, it is a really good indicator.
> It can be better than using the L test or others that compare reflections because it does not require that you have the correct symmetry.
On 7 September 2012 09:10:19 GMT+01:00 Randy Read wrote:
>> Twinning tests that compare the intensities of potentially twin-related reflections to see if they are more closely related than expected randomly (e.g. the H test or the Britton plot) can't tell the difference between twinning and either higher symmetry or pseudosymmetry. So you need to use a test based on intensity distributions independent of twin laws (like the intensity moments or the L test) to decide if your crystal is twinned, then if it is twinned you will get a better idea of the twin fraction from tests comparing the twin-related reflections.
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