Hi Eleanor, Are you sure about the application of twinning tests to look for lattice-translocation defects? In twins the intensities sum, but in lattice translocation cases, the domains can be so small that you can't simply sum intensities . . . at least that's how I interpret http://www.amercrystalassn.org/HotNews/twinning/yeates.pdf See the summary on slide 41 in particular.
Pete ****************** Hmm - that is hard.. pseudo translation is relatively harmless - you have 2 or more molecules in the same orientation but in different positions in the unit cell, and the structure factors they generate will have some different properties. For instance the 0k0 in your case will always have k=2n+1 weak because the translation is xt, 0.5,zt ( you can work that out from a SF equation if you like!) And since the xt = 0.02, ie is rather small, and zt = 0, at low resolution all the hkl with k=2n+1 will be weak. use hklview data.mtz and look at h0l then next level , next level etc and you should see this effect.. The only problem it gives in is determining the spacegroup. But phaser will usually sort that out as long as you let it test all SGs . Lattice translation is effectively one form of twinning, you can visualise it as a set of crystals where that lattice are aligned in 2 dimensions but there is slippage along the third. So each "reflection" is in fact the sum of two or more intensities and the twinning analyses should be valid. But as well you have the problem that some classes of reflections are very weak, in the same way as a pseudo translation affects the data. And the twinning tests via moments, H test and Britten test are all distorted by the weak/strong pattern so really the only effective test is the L test, and that too can be badly distorted by anisotropy and other defects. Apparently it is often possible to recognise a lattice defect by looking at the images, if you are good at that. Some classes of reflections will be very streaky ( where there is an overlap between the different crystal fragments) and others sharp. But once the data is integrated that information is lost. Does that help? Eleanor
