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/
