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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