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.
George Prof. George M. Sheldrick FRS Dept. Structural Chemistry, University of Goettingen, Tammannstr. 4, D37077 Goettingen, Germany Tel. +49-551-39-3021 or -3068 Fax. +49-551-39-22582 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.. > > Eleanor > > Serge Cohen wrote: > > -----BEGIN PGP SIGNED MESSAGE----- > > Hash: SHA1 > > > > 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 > > > > -----BEGIN PGP SIGNATURE----- > > Version: GnuPG v1.4.8 (Darwin) > > > > iEYEARECAAYFAkk5KCgACgkQlz6UVQtc2uy09ACg69gIYn0Lc1e2tSRzoi1EJOe9 > > g/gAoOplij/XAxmGoZaNjFjAUx/etD/1 > > =QpB8 > > -----END PGP SIGNATURE----- > > > > > > > >
