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Actually, the thing that matters for non-isomorphism is the absolute value of the change in cell dimension compared to the resolution of the data. A change of 2A is half dmin in this case, so there will be a significant impact on isomorphism in the highest resolution shells (which should also be seen in statistics from aimless). To give credit appropriately, I first heard the argument for the absolute change being important in a lecture from Kevin Cowtan, where he was turning it around and saying that the size of cell dimension change determines whether you will get any phase information from multi-crystal averaging (where you *don't* want the crystals to be isomorphous). However, I think the same point was made in some of the earlier literature on MIR, before people (certainly not just Harry — probably more like the majority!) got stuck on the idea that the percentage change was the way to look at it. Best wishes, Randy Read > On 30 Sep 2015, at 10:49, Harry Powell <[log in to unmask]> wrote: > > Hi > >>> >>> I have a few datasets from multiple crystals with unit cells of about 440, 440, 80 A (90, 90, 120 degrees; point group P 622). While the best 3 clusters together well in BLEND, there is one that does not as its a and b axes are 2 A longer. For the purpose of merging them, can they be considered isomorphic to each other? > > Possibly - 0.5% difference in cell edges is not *too* much. I’d try it and see… > > Harry > -- > Dr Harry Powell, MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge Biomedical Campus, Cambridge CB2 0QH > Chairman of International Union of Crystallography Commission on Crystallographic Computing > Chairman of European Crystallographic Association SIG9 (Crystallographic Computing) > >>> On 30 Sep 2015, at 08:59, Mohamed Noor <[log in to unmask]> wrote: >>> >>> Dear all >>> >>> I have a few datasets from multiple crystals with unit cells of about 440, 440, 80 A (90, 90, 120 degrees; point group P 622). While the best 3 clusters together well in BLEND, there is one that does not as its a and b axes are 2 A longer. For the purpose of merging them, can they be considered isomorphic to each other? >>> >>> There is an extensive radiation damage (initial resolution from the first 20 degrees is about 3.8 A), so I am hoping to get a good complete dataset. >>> >>> Thanks. >>> Mohamed ------ Randy J. Read Department of Haematology, University of Cambridge Cambridge Institute for Medical Research Tel: + 44 1223 336500 Wellcome Trust/MRC Building Fax: + 44 1223 336827 Hills Road E-mail: [log in to unmask] Cambridge CB2 0XY, U.K. www-structmed.cimr.cam.ac.uk