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Going back to the original discussion on XDS producing negative CCanom, I am not sure that this is the result of some crystal artifact. I just checked many datasets from the last years and in all cases virtually for every every resolution shell, XDS reports a negative CC anom. Please note that this is not the case in cases where an anomalous signal is present, in those cases the signal drops to zero smoothly and occasionally at high resolution a CC anom of -1% pops up. I am talking of cases where there is no significant anomalous signal. Then it is reported negative and often "higher" (lower) than -10%. I must say that my typical data collection and data processing involves pushing a bit both with resolution and with radiation dose, so maybe this is partly the reason. IMO this is XDS trying to make the best of a signal that isn't there and turning zero + noise into zero + "negative" noise somehow. Just again, if there is a significant anomalous signal, then CCanom behaves as one would expect, positive at low res, decaying at medium res and 0+-noise at high res. Cheers, Jose. ================================ Jose Antonio Cuesta-Seijo, PhD Carlsberg Laboratory Gamle Carlsberg Vej 10 DK-1799 Copenhagen V Denmark Tlf +45 3327 5332 Email [log in to unmask] ================================ -----Original Message----- From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of Kay Diederichs Sent: Friday, July 17, 2015 8:00 AM To: [log in to unmask] Subject: Re: [ccp4bb] Negative CCanom Am 17. Juli 2015 00:00:11 MESZ, schrieb "Keller, Jacob" <[log in to unmask]>: >>Jacob's case is twinning in P3(2)12 making the data appear as P6(2)22, >so it is indeed a rotation by 180°. > >Yes, this all fits together nicely. If I understand correctly, this >would make my crystals a blend of mero- and pseudo-mero-hedral, so >involving I's and F's, no? No, it's just merohedral - the true and the apparent spacegroup belong to the same pointgroup. Even if it were pseudo-merohedral it would be a superposition of reflections, with addition of their intensities. HTH, Kay > > > >JPK -- Diese Nachricht wurde von meinem Android-Mobiltelefon mit K-9 Mail gesendet.