[phenixbb] pseudo-merohedral twinning or pseudo-symmetry?

Laurie Betts laurie.betts0508 at gmail.com
Fri Oct 16 12:20:20 PDT 2009

Hi - We have a data set that seems to process well in primitive
orthorhombic.  a=80, b=87, c=114.  The data  has overall Rsymm around
10% to 2.6 Angstrom, where I/sigI > 2, and Chi-squared is around 1.4.
Phenix.xtriage shows some pathology in intensity statistics such that
the L-test indicates <|L|> = 0.414 and <L^2> = 0.241, with the L-test
Z score = 7.334.  The Wilson ratio for acentric refls is 1.774.
According the xtriage, There were no "pseudomerohedral" twin laws
found, and the comments at the end say "As there are no twin laws
possible given the crystal symmetry, there could be a number of

Is it more likely that this is a case of pseudo-symmetry, or would one
not expect the intensity statistics to have the values they do in that

Also, is the absence of twin laws defined by the fact that proper
merohedral twinning is not possible for P222 space groups?

Still confused,

Laurie Betts

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