Hi Harry, I remember the C222/P622 confusion from some of Phil's old slides, and I thought that was an impressive case! It probably appears in a few places on the internet, but one I found just now is slide 15 of this presentation: https://www.ccp4.ac.uk/schools/DLS-2015/course_material/Datareduction2015.pdf
Cheers -- David On Fri, 23 Feb 2024 at 10:17, Harry Powell < 0000193323b1e616-dmarc-requ...@jiscmail.ac.uk> wrote: > Hi Graeme (I may well have mentioned this when we shared an office) et al > > I’ll add something from my small-molecule crystallography days, when we > used point detectors - so this would be pre-1996 which was the last time I > used one of these machines. > > I don’t remember which structure it was (feel free to go through the CSD > to check on my behalf, but many structures were not deposited in those days > and languish in a PhD thesis!); I had a dataset with three 90º angles, but > the processing statistics (and overall cell volume) indicated quite plainly > that it was monoclinic (probably P21). I re-refined the unit cell as if it > were triclinic and the “best” 90 degree angle with the smallest ESD was the > one that corresponded to the monoclinic beta; the two 90º angles refined > away from their true value more. > > A result of that experiment was that (since then) I never assumed that the > values of the angles from the data processing showed unambiguously that I > had a high symmetry solution. I believe that Pointless arose after a > hexagonal/C-centred orthorhombic ambiguity arose (but my memory could be > faulty here). > > Best wishes > > Harry > > > On 23 Feb 2024, at 09:58, Winter, Graeme (DLSLtd,RAL,LSCI) < > 00006a19cead4548-dmarc-requ...@jiscmail.ac.uk> wrote: > > > > Hi Huw > > > > (first: thank to Phil for picking this up; it caused much confusion) > > > > While I get where you are coming from, it is still from a mathematical > standpoint correct to consider e.g. a tetragonal crystal as monoclinic - > P21 is a subgroup of P43212 (say) so strictly it is possible and correct - > if experimentally unlikely - to have the situation we are discussing here > occur. > > > > Also, under merging data to investigate twinning is a current bb topic. > > > > Telling users to “fiddle the parameters” so that the strict test is > satisfied feels like a non-ideal answer: a warning when importing such data > could be legitimate e.g. “hmm I note a = b and al=be=ga=90 _exactly_ this > is unusual, I hope you know what you are doing” rather than a flat out > error. > > > > Literally I got involved as I had a dials user ask me how to do this > parameter fiddling in a more niche case and I thought that was a suboptimal > solution to an artificial problem :-) > > > > On a personal note, I think it is important that the tools we develop > still allow people to explore problems rather than railroading them down > one true route which is the only allowed way to look at a problem: we learn > a lot by exploring odd corners as here. > > > > Best wishes Graeme > > > >> On 23 Feb 2024, at 09:49, Huw Jenkins <h.t.jenk...@me.com> wrote: > >> > >> [You don't often get email from h.t.jenk...@me.com. Learn why this is > important at https://aka.ms/LearnAboutSenderIdentification ] > >> > >> Hi Graeme, > >> > >>> On 21 Feb 2024, at 16:52, Winter, Graeme (DLSLtd,RAL,LSCI) < > 00006a19cead4548-dmarc-requ...@jiscmail.ac.uk> wrote: > >>> > >>> Processing a data set in lower than necessary symmetry e.g. tetragonal > as monoclinic you _cannot_ import the merged MTZ file into i2 because it is > impossible to have 90 degree angles for P21 > >> > >> I had a look at the code in CCP4i2 that generates the errors in the > screenshots you posted. The first one is only generated if two cell > parameters are *exactly* equal and the second is generated when beta is > between 89.9999 and 90.0001 degrees. > >> > >> I think these tests should only fail if the data were processed > assuming higher symmetry so that unit cell parameters were restrained and > then the space group changed to a lower symmetry one. Isn't the correct > approach when the true symmetry is lower than originally assumed to repeat > the data processing without applying constraints imposed by the higher > symmetry - because, for example, cell parameters refined assuming cell > length/angle constraints may not predict the reflection positions as well > as if these restraints were not applied, reflections assumed to be symmetry > equivalent when they weren't may lead to suboptimal scaling etc etc? > >> > >> > >> Huw > > > > > > -- > > This e-mail and any attachments may contain confidential, copyright and > or privileged material, and are for the use of the intended addressee only. > If you are not the intended addressee or an authorised recipient of the > addressee please notify us of receipt by returning the e-mail and do not > use, copy, retain, distribute or disclose the information in or attached to > the e-mail. > > Any opinions expressed within this e-mail are those of the individual > and not necessarily of Diamond Light Source Ltd. > > Diamond Light Source Ltd. cannot guarantee that this e-mail or any > attachments are free from viruses and we cannot accept liability for any > damage which you may sustain as a result of software viruses which may be > transmitted in or with the message. > > Diamond Light Source Limited (company no. 4375679). Registered in > England and Wales with its registered office at Diamond House, Harwell > Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, United Kingdom > > > > > > ######################################################################## > > > > To unsubscribe from the CCP4BB list, click the following link: > > https://www.jiscmail.ac.uk/cgi-bin/WA-JISC.exe?SUBED1=CCP4BB&A=1 > > > > This message was issued to members of www.jiscmail.ac.uk/CCP4BB, a > mailing list hosted by www.jiscmail.ac.uk, terms & conditions are > available at https://www.jiscmail.ac.uk/policyandsecurity/ > > ######################################################################## > > To unsubscribe from the CCP4BB list, click the following link: > https://www.jiscmail.ac.uk/cgi-bin/WA-JISC.exe?SUBED1=CCP4BB&A=1 > > This message was issued to members of www.jiscmail.ac.uk/CCP4BB, a > mailing list hosted by www.jiscmail.ac.uk, terms & conditions are > available at https://www.jiscmail.ac.uk/policyandsecurity/ > ######################################################################## To unsubscribe from the CCP4BB list, click the following link: https://www.jiscmail.ac.uk/cgi-bin/WA-JISC.exe?SUBED1=CCP4BB&A=1 This message was issued to members of www.jiscmail.ac.uk/CCP4BB, a mailing list hosted by www.jiscmail.ac.uk, terms & conditions are available at https://www.jiscmail.ac.uk/policyandsecurity/
