Error estimates for the unit cell dimensions in macromolecular crystallography belong to atypical category of uncertainty estimates.
Random error contribution in most cases is below 0.001A, so it can be neglected. Wavelength calibration error can be also made very small; however, I do not know how big it is in practice. Goniostat wobble error is taken into account in Scalepack refinement. Crystal-to-detector distance is not used in postrefinement/global refinement. Due to the measurement error being very small, even small variations in unit cell parameters can be detected within cryocooled crystals. These variations almost always are _orders_of_magnitude_larger_ than measurement uncertainty. Current practise is not to investigate the magnitude of the changes in the unit cell parameters, but when beam smaller than crystal is used, observing variations as large as 1A is not unusual. The main question is: what the unit cell uncertainty means? For most samples I could defend to use values: 0.001A, 0.01A, 0.1A and 1A as reasonable, depending on particular point of view. Without defining what the unit cell uncertainty means, publishing its values is pointless. Zbyszek Otwinowski > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > Hi Bernhard, > > A look at the methods section might give you a clue. Neither XDS nor > XSCALE create mmCIF - files (you are talking about mmCIF, not CIF - > subtle, but annoying difference), so that the choice is limited. I > guess some programmer (rather than a scientist ;-) )used a simple > printf commmand for a double precision number so the junk is left over > from the memory region or other noise common to conversions. > > XDS actually prints error estimates for the cell dimensions in > CORRECT.LP which could be added to the mmCIF file - a cif (sic!) file, > I believe, requires those, by the way and checkCIF would complain > about their absence. > > Cheers, > Tim > > On 07/22/2014 01:01 PM, Bernhard Rupp wrote: >> I am just morbidly curious what program(s) deliver/mutilate/divine >> these cell constants in recent cif files: >> >> >> >> data_r4c69sf >> >> # >> >> _audit.revision_id 1_0 >> >> _audit.creation_date ? >> >> _audit.update_record 'Initial release' >> >> # >> >> _cell.entry_id 4c69 >> >> _cell.length_a 100.152000427 >> >> _cell.length_b 58.3689994812 >> >> _cell.length_c 66.5449981689 >> >> _cell.angle_alpha 90.0 >> >> _cell.angle_beta 99.2519989014 >> >> _cell.angle_gamma 90.0 >> >> # >> >> >> >> Maybe a little plausibility check during cif generation might be >> ok >> >> >> >> Best, BR >> >> >> >> PS: btw, 10^-20 meters (10^5 time smaller than a proton) in fact >> seriously challenges the Standard Model limits.. >> >> ---------------------------------------------------------------------------- >> >> > - ------------ >> >> Bernhard Rupp >> >> k.-k. Hofkristallamt >> >> Crystallographiae Vindicis Militum Ordo >> >> b...@ruppweb.org >> >> b...@hofkristallamt.org >> >> http://www.ruppweb.org/ >> >> ----------------------------------------------------------------------- >> >> >> >> >> >> >> > > - -- > - -- > Dr Tim Gruene > Institut fuer anorganische Chemie > Tammannstr. 4 > D-37077 Goettingen > > GPG Key ID = A46BEE1A > > -----BEGIN PGP SIGNATURE----- > Version: GnuPG v1.4.12 (GNU/Linux) > Comment: Using GnuPG with Icedove - http://www.enigmail.net/ > > iD8DBQFTzk52UxlJ7aRr7hoRAul8AKCHFz/DAoqR7s0fGUp79xx2QlrfCQCeIiiy > KXSurhgaQjhguKr9L0/zyVk= > =vqGC > -----END PGP SIGNATURE----- > Zbyszek Otwinowski UT Southwestern Medical Center at Dallas 5323 Harry Hines Blvd. Dallas, TX 75390-8816 Tel. 214-645-6385 Fax. 214-645-6353
