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
