Re: [ccp4bb] units of the B factor

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From: Frank von Delft frank {- dot -} vondelft {- at -} SGC {- dot -} OX {- dot -} AC {- dot -} UK
Date: 2009-11-20

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Subject: Re: units of the B factor
From: Adam Ralph aralph {- at -} MATHS {- dot -} NUIM {- dot -} IE
Date: 2009-11-20

I think that you should suggest a new unit of 10^(-11) m, a JHm
perhaps. If it is convenient to have B in A^2 then u^2 should be
in JHm^2.

Adam

On Thu, 19 Nov 2009, James Holton wrote:

> 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
>

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Previous message:
Subject: {Spam?} Re: {Spam?} Re: units of the B factor
From: Frank von Delft frank {- dot -} vondelft {- at -} SGC {- dot -} OX {- dot -} AC {- dot -} UK
Date: 2009-11-20

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