[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [ccp4bb]: Tetragonal Twinning & Detwin
*** For details on how to be removed from this list visit the ***
*** CCP4 home page http://www.ccp4.ac.uk ***
On Wed, 19 Sep 2001, David Briggs wrote:
> *** For details on how to be removed from this list visit the ***
> *** CCP4 home page http://www.ccp4.ac.uk ***
>
> Dear ccp4-ers...
>
> I have a dataset that I scales equally well in P4 and P422. In
> order to resolve this ambiguity I looked at the p4 scaled data in hklview
> and found mirror planes in all the right places suggesting that the laue
> class was 4/m mm, therefore p422.
> All the moments and intensity statistics in Scala/truncate look fine when
> the data is scaled in p422, but not quite as good in p4.
> Just for the hell of it, I ran the detwin program on the p4 scaled data,
> and detwin reckons my data is pretty much a perfect twin.
> So... if the true space group is p422, and you put p4 data through detwin,
> will it appear twinned? (as 'perfectly' twinned p4 data can appear to be
> p422...)
Depending on how the twinning server works, the bad news probably is that your
data is twinned. Processing P422 data as P4 does not change the intensity
distribution, there will be just pairs of symmetry related reflections with
very similar intensities that didn't get merged together. Only if the twinning
server compares twin-related intensities it could be fooled into thinking your
crystal is seriously twinned.
Take a look at your TRUNCATE (there it is again) output to see if your
intensity (or better E-value) distributions are normal or not. If they
indicate twinning then I fear you do have a problem.
Bart
===============================================================================
Dept. of Medical Microbiology & Immunology
University of Alberta
1-15 Medical Sciences Building
Edmonton, Alberta, T6G 2H7, Canada
phone: 1-780-492-0042
fax: 1-780-492-7521
===============================================================================