Indeed that paper does lay out clearly the various definitions, thank you, but I note that you do explicitly discourage use of B (= 8 pi^2 U), and don't explain why the factor is 8 rather than 2 (ie why it multiplies (d*/2)^2 rather than d*^2). I think James Holton's reminder that the definition dates from 1914 answers my question.
So why do we store B in the PDB files rather than U? :-) Phil On 12 Oct 2011, at 21:19, Pavel Afonine wrote: > This may answer some of your questions or at least give pointers: > > Grosse-Kunstleve RW, Adams PD: > On the handling of atomic anisotropic displacement parameters. > Journal of Applied Crystallography 2002, 35, 477-480. > > http://cci.lbl.gov/~rwgk/my_papers/iucr/ks0128_reprint.pdf > > Pavel > > On Wed, Oct 12, 2011 at 6:55 AM, Phil Evans <p...@mrc-lmb.cam.ac.uk> wrote: > I've been struggling a bit to understand the definition of B-factors, > particularly anisotropic Bs, and I think I've finally more-or-less got my > head around the various definitions of B, U, beta etc, but one thing puzzles > me. > > It seems to me that the natural measure of length in reciprocal space is d* = > 1/d = 2 sin theta/lambda > > but the "conventional" term for B-factor in the structure factor expression > is exp(-B s^2) where s = sin theta/lambda = d*/2 ie exp(-B (d*/2)^2) > > Why not exp (-B' d*^2) which would seem more sensible? (B' = B/4) Why the > factor of 4? > > Or should we just get used to U instead? > > My guess is that it is a historical accident (or relic), ie that is the > definition because that's the way it is > > Does anyone understand where this comes from? > > Phil >
