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Twinning: Difference between revisions
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== Solution of twinned structures by experimental phasing == | |||
== Solution == | |||
[http://dx.doi.org/10.1107/S0108767387099951 Yates & Rees, Acta Cryst. (1987). A43, 30-36] give a method for solving a twinned structure using MIR. This requires four independent derivatives. | [http://dx.doi.org/10.1107/S0108767387099951 Yates & Rees, Acta Cryst. (1987). A43, 30-36] give a method for solving a twinned structure using MIR. This requires four independent derivatives. | ||
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Refinement of merohedrally and pseudo-merohedrally twinned data data is possible using [[CNS]] (for input files, see CNS homepage - main menu - input files - x ray - twinning), [[ccp4dev:Refinement_with_Refmac5|Refmac]] or [[Phenix|phenix.refine]] ([http://www.phenix-online.org/documentation/refinement.htm#anch38 Refinement using twinned data]) and has "always" been possible with [[SHELXL]] ([http://shelx.uni-ac.gwdg.de/~rherbst/twin.html Twin-Refinement with SHELXL]). | Refinement of merohedrally and pseudo-merohedrally twinned data data is possible using [[CNS]] (for input files, see CNS homepage - main menu - input files - x ray - twinning), [[ccp4dev:Refinement_with_Refmac5|Refmac]] or [[Phenix|phenix.refine]] ([http://www.phenix-online.org/documentation/refinement.htm#anch38 Refinement using twinned data]) and has "always" been possible with [[SHELXL]] ([http://shelx.uni-ac.gwdg.de/~rherbst/twin.html Twin-Refinement with SHELXL]). | ||
Be aware that R-factors may not be directly comparable: [http://www.ysbl.york.ac.uk/refmac/papers/Rfactor.pdf G. Murshudov, Appl. Comput. Math., V.10, N.2, 2011, pp.250-261] and [http://journals.iucr.org/d/issues/2013/07/00/ba5190/index.html Evans P.R., Murshudov G.N. (2013) How good are my data and what is the resolution? Acta Cryst D69, 1204-1214] | Be aware that R-factors may not be directly comparable: [http://www.ysbl.york.ac.uk/refmac/papers/Rfactor.pdf G. Murshudov, Appl. Comput. Math., V.10, N.2, 2011, pp.250-261] (see figure) | ||
[[File:G. Murshudov, Appl. Comput. Math., V.10, N.2, 2011, pp.250-261 Fig3.png|thumb]] | |||
and [http://journals.iucr.org/d/issues/2013/07/00/ba5190/index.html Evans P.R., Murshudov G.N. (2013) How good are my data and what is the resolution? Acta Cryst D69, 1204-1214] | |||
== '''Warning Signs for Twinning''' == | == '''Warning Signs for Twinning''' == | ||
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# Some reflections are sharp, others split. | # Some reflections are sharp, others split. | ||
# K = mean(Fo^2)/mean(Fc^2) is systematically high for reflections with low intensity. This may also indicate a wrong choice of space group in the absence of twinning. | # K = mean(Fo^2)/mean(Fc^2) is systematically high for reflections with low intensity. This may also indicate a wrong choice of space group in the absence of twinning. | ||
== References == | == References == | ||