Re: [ccp4bb]: accuracy and precision of unit cell axes

[Mischa Machius]

In my experience, both cell parameters and mosaicity, as well as other parameters, can vary quite a bit during data collection of a single crystal, particularly for synchrotron data. As a result, there is no single set of cell parameters that satisfactorily describes a given crystal in all orientations and at all stages during data collection. They are clearly not cell "constants". When refining cell parameters and mosaicity in batches of, say, five frames and then simply calculating average values plus standard deviations, I invariably find a larger spread than that reported in the HKL2000 GUI. This method is probably not entirely accurate, but it demonstrates the variability. So, for describing a particular crystal, a single decimal should be appropriate. For describing a crystal form as a whole, specifying approximate cell parameters, say 120 x 105 x 97 Å, not even bothering with decimal points, should be appropriate, because cell parameters can also vary significantly between crystals. Best - MM On Jun 5, 2006, at 12:34 PM, Yoder, Marilyn wrote: Hi all,   I have a reviewer challenging the number of digits to the right of the decimal point that I have reported in a submitted manuscript.  I had reported 3 digits to the right, frankly because that is what most of the programs I use report and the PDB coordinate file reports 3 digits to the right of the decimal in the CRYST1 record.  The reviewer doubts that the value is accurate to that many digits, and suggested reporting to the tenth of an Angstrom.  In corresponding with the PDB, they allow 3 digits to the right of the decimal, but even if one gives the PDB, say 51.1 A as an axis, the CRYST1 record will be padded and will appear as 51.100A.   So my question is .... how does one determine what is a valid accuracy or precision in unit cell axes?  To be honest, I'm not sure what all the numbers are, and how many significant digits they have, that are used in calculating unit cell axes.    In my particular case, I have data to 1.73 A, synchrotron radiation, 0.41 degree mosacity.  All unit cells are less than 115 A.    Thanks for any insight.   Regards, Marilyn   Marilyn D. Yoder Division of Cell Biology & Biophysics School of Biological Sciences University of Missouri-Kansas City 5007 Rockhill Rd. Kansas City, MO 64110-2499 phone 816-235-1503 fax 816-235-1503   -------------------------------------------------------------------------------- Mischa Machius, PhD Associate Professor UT Southwestern Medical Center at Dallas 5323 Harry Hines Blvd.; ND10.214A Dallas, TX 75390-8816; U.S.A. Tel: +1 214 645 6381 Fax: +1 214 645 6353