Of course, for SI political correctness we should be using nm^2 anyway. This would add more confusion to a situation that most people don't worry about anyway.
Pete On 20 Nov 2009, at 11:05, Ian Tickle wrote: > Hi James > > If we're going to sort out the units we need to get the terminology > right too. The mean square atomic displacement already has a symbol U = > <u^2> (or to be precise Ueq as we're talking about isotropic > displacements here), and u is conventionally not defined as the RMS > displacement as you seem to be implying, but the *instantaneous* > displacement (otherwise you then need another symbol for the > instantaneous displacement!). > > See: > http://www.iucr.org/resources/commissions/crystallographic-nomenclature/ > adp > (or Acta Cryst. (1996). A52, 770-781). > > My theory is that B became popular over U because it needs 1 fewer digit > to express it to a given precision, and this was important given the > limited space available in the 80-column PDB format. So a B of 20.00 to > 4 sig figs requires 5 columns, whereas the equivalent U of 0.2500 to 4 > sig figs requires 6 columns (personally I've got nothing against '.2500' > but many compiler writers don't see it my way!). > > Interestingly the IUCr commission in their 1996 report did not address > the question of units for B and U. > > Cheers > > -- Ian > >> -----Original Message----- >> From: owner-ccp...@jiscmail.ac.uk >> [mailto:owner-ccp...@jiscmail.ac.uk] On Behalf Of James Holton >> Sent: 20 November 2009 07:14 >> To: CCP4BB@jiscmail.ac.uk >> Subject: units of the B factor >> >> Many textbooks describe the B factor as having units of >> square Angstrom >> (A^2), but then again, so does the mean square atomic >> displacement u^2, >> and B = 8*pi^2*u^2. This can become confusing if one starts >> to look at >> derived units that have started to come out of the radiation damage >> field like A^2/MGy, which relates how much the B factor of a crystal >> changes after absorbing a given dose. Or is it the atomic >> displacement >> after a given dose? Depends on which paper you are looking at. >> >> It seems to me that the units of "B" and "u^2" cannot both be A^2 any >> more than 1 radian can be equated to 1 degree. You need a scale >> factor. Kind of like trying to express something in terms of "1/100 >> cm^2" without the benefit of mm^2. Yes, mm^2 have the >> "dimensions" of >> cm^2, but you can't just say 1 cm^2 when you really mean 1 mm^2! That >> would be silly. However, we often say B = 80 A^2", when we >> really mean >> is 1 A^2 of square atomic displacements. >> >> The "B units", which are ~1/80th of a A^2, do not have a name. So, I >> think we have a "new" unit? It is "A^2/(8pi^2)" and it is >> the units of >> the "B factor" that we all know and love. What should we call it? I >> nominate the "Born" after Max Born who did so much fundamental and >> far-reaching work on the nature of disorder in crystal lattices. The >> unit then has the symbol "B", which will make it easy to say >> that the B >> factor was "80 B". This might be very handy indeed if, say, >> you had an >> editor who insists that all reported values have units? >> >> Anyone disagree or have a better name? >> >> -James Holton >> MAD Scientist >> >> > > > Disclaimer > This communication is confidential and may contain privileged information > intended solely for the named addressee(s). It may not be used or disclosed > except for the purpose for which it has been sent. If you are not the > intended recipient you must not review, use, disclose, copy, distribute or > take any action in reliance upon it. If you have received this communication > in error, please notify Astex Therapeutics Ltd by emailing > i.tic...@astex-therapeutics.com and destroy all copies of the message and any > attached documents. > Astex Therapeutics Ltd monitors, controls and protects all its messaging > traffic in compliance with its corporate email policy. The Company accepts no > liability or responsibility for any onward transmission or use of emails and > attachments having left the Astex Therapeutics domain. Unless expressly > stated, opinions in this message are those of the individual sender and not > of Astex Therapeutics Ltd. The recipient should check this email and any > attachments for the presence of computer viruses. Astex Therapeutics Ltd > accepts no liability for damage caused by any virus transmitted by this > email. E-mail is susceptible to data corruption, interception, unauthorized > amendment, and tampering, Astex Therapeutics Ltd only send and receive > e-mails on the basis that the Company is not liable for any such alteration > or any consequences thereof. > Astex Therapeutics Ltd., Registered in England at 436 Cambridge Science Park, > Cambridge CB4 0QA under number 3751674
