[ccp4bb]: Tetragonal Twinning and Detwin - solution?

[David Briggs <d.briggs@mail.cryst.bbk.ac.uk>]

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Dear all....

	Following on from my problems regarding tetragonal twinning and
some ambiguity between P4 (twinned) and P422 (non-twinned), we took an
un-scaled mtz file from a solved/published structure from our college that
was solved in P422 (4/m mm). This integrated mtz file was in p4.

	We then re-indexed this in p422 and repeated scala/truncate/detwin
on both P4 and P422 datasets.

	Both my data and the solved data scale equally well in P4 and P422
(sensible stats, very few rejections...)

	The P4 centric intensity distribution was also a little odd, where
as the p422 looks fine. All the various moments in P4 and P422 indicated
that the data was not twinned.
	Detwin also indicated that, in P4, this data was an almost perfect
twin. The UCLA perfect twinning test for P422 indicated "no twin", but the
partial test in P4 indicated almost perfect twin.

At some point I will put all the new graphs on the webpage...

http://student.cryst.bbk.ac.uk/~ebrig02/twin/

	As this structure has been solved to about 2.8A, it is fairly
safe to assume that it is not twinned...
(CLANG!!, famous last words etc...)

When data scales equally well in both higher and lower space groups,
provided that there are NO indications of twinning in both intensity
distribution AND moments, then is it safe to assume that it isn't twinned,
and it IS the higher symmetry, despite the fact that Twinning tests indicate
that the lower symmetry is almost perfectly twinned? (making us believe that
the higher symmetry is an artifact of the merohedral twinning).... (!)

	 Therefore, for near perfect twinning, should one pay more
attention to the UCLA "perfect Twinning Test" than other tests designed
for partial twinning?

	Cheers

		Dave

	P.S. So maybe my crystal isn't twinned...?

	P.P.S (CLANG!)
_______________________________________________________________________________
Dave Briggs
School of Crystallography
Birkbeck College
Malet Street
Bloomsbury
London
WC1E 7HX