Such anomalous <I*2>/<I>*2 values usually reflect the fact that you have
a non-crystllographic translation which means a large % of your
reflections will be very weak ( eg if you had mistakenly called a C2
data set P2, all the h+k=2n+1 would be ~0, and non-crystalllographic
translation can cause similar near-absences)
However the Ltest for twinning is less affected by such a
non-crystallographic translation and can indicate twinning.
Programs also use the H-test that looks at correlations between
reflections which ould be matched by a twin operator. This is a good
test PROVIDING you have the right point group.. If for instance you give
your pointgroup as PG4 when it is really PG422 the Htest will say -
brilliant match between hkl andkh-l; maybe that is a twin operator with
alpha 0.49999
I guess all this adds up to the fact that you need to be cautious re
twinning..
Eleanor
Ben Flath wrote:
I have used phenix.xtriage. It finds three twin laws with alpha ~0.48 for all of them and the results of the perfect twin test are well over 2.
Ben
On 9/22/09 1:50 PM, Tanner, John J. wrote:
I suggest you analyze your data with phenix.xtriage. It does several tests and
gives descriptive output.
________________________________
From: Ben Flath <bef...@mail.usask.ca>
Reply-To: Ben Flath <bef...@mail.usask.ca>
Date: Tue, 22 Sep 2009 14:31:26 -0500
To: <CCP4BB@JISCMAIL.AC.UK>
Conversation: [ccp4bb] perfect twin test
Subject: [ccp4bb] perfect twin test
Hi all
when subjecting my data to the perfect twin test <I*2>/<I>*2 I get values very
close to 3 which is far off the theoretical values of 1.5 and 2 for twinned and untwinned
data.
can anyone shed some light on what might be going on with my data. Could
tetartohedral twinning have anything to do with it?
Thanks
---------------------------------
Benjamin Flath
College of Pharmacy and Nutrition
University of Saskatchewan
320 Thorvaldson Building
110 Science Place
Saskatoon, SK
S7N 5C9
