Re a commensurate superlattice case Dear CCP4bb Colleagues,
Thanks to the varied interest and especially even as far as browsing and processing of diffraction images kindly undertaken by Kay Diederichs, Gloria Borgstahl and Jeff Lovelace, I went back to one-by-one scrutiny of images including another native that I had not done that for so far. I finally found a zone axis which autoindexed in P2 (now with the problem axis labelled as 'c', just a labelling effect of course). With this now known to me I found that in the main native data set and a Pb data set that I could just about see the same zone axis ie much more misset but capable of indexing in the same P2 cell but with quite strong non-isomorphism (~3%). Anyway I have provided details to Gloria Borgstahl for a grant proposal (update) she is making imminently. So, whilst oddity 1, defined in my first email ie 'triclinic but with alpha approx = 90 degrees', I regard as solved oddity 2, 'the commensurate cell doubling effect', is still insurmountable at present. Nb there can be favourable commensurate cases such as pointed out to me by Ethan Merritt but where non-isomorphism did not strike too. Apologies to those colleagues that I have not highlighted here, but who were all helpful either with ingenious suggestions or with good strategic/tactical advice. Best wishes, John Helliwell -- John R Helliwell Professor of Structural Chemistry, The University of Manchester;
