A good compromise in the case of suspected perfect merohedral twinning
is to select the free R reflections using thin shells. Since the twin
related reflections have the same sin(theta)/lambda values they will
not be split over the working and reference sets. XPREP offers this
option. A disadvantage is the the maps may not be quite as good as
when the free R reflections are selected randomaly.
Prof. George M. Sheldrick FRS
Dept. Structural Chemistry,
University of Goettingen,
D37077 Goettingen, Germany
Tel. +49-551-39-3021 or -3068
On Mon, 8 Dec 2008, Eleanor Dodson wrote:
> you need to select the Free R in the highest possible Laue group , then use
> cad to extend this to all reflections in the chosen point group. This is best
> done immediately post-truncate..
> eg if you twinning operator is k,h,-l, then if
> 1,2, 3 is assigned to the free set so should 2, 1, -3 be.
> PHENIX.xtriage does it properly
> REFMAC does not use it properly I dont think, but maybe that has been fixed..
> Serge Cohen wrote:
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> > I guess the independence is mostly depending on how you select the
> > reflection that lie in the free-set.
> > If (from the beginning) you select reflections that are related by the twin
> > operator to be both in the free-set you keep the "independence" of the
> > free-set.
> > Serge.
> > Le 5 déc. 08 à 13:50, Agnieszka Kiliszek a écrit :
> > > Hi,
> > >
> > > What is the meaning of R free of perfect twin? Shelxl counts it but it
> > > seems that the R f is no longer independent.
> > >
> > > Aga
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