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Dear James, I agree with Pavel that your example is not very realistic. In practice one would start from the heavy atom positions. As well as providing starting phases, they are useful in other ways. For example. shelxe (and probably most other tracing programs) adds them to a 'no-go' map so it knows where NOT to trace the main-chain. Best wishes, George > Dear James, > > your challenge in its current form ignores an important source > of information for model building that is available for your > simulated data - namely, it does not allow to use anomalous > phase information in the model building. In difficult cases on > the edge of success such as this one, this typically makes > the difference between building and not building. > > If you can make the F+/F- and Se substructure available, we > can test whether this is the case indeed. However, while I > expect this would push the challenge further significantly, > most likely you would be able to decrease the Se incorporation > of your simulated data further to such levels that the anomalous > signal is again no longer sufficient to build the structure. And > most likely, there would again exist an edge where a small > decrease in the Se incorporation would lead from a model built > to no model built. > > Best regards, > > -- > Pavol Skubak > Biophysical Structural Chemistry > Gorleaus Laboratories > Einsteinweg 55 > Leiden University > LEIDEN 2333CC > the Netherlands > tel: 0031715274414 <tel:0031715274414> > web: http://bsc.lic.leidenuniv.nl/people/skubak-0 -- 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