Hi Nick
The 0.1 to 0.3 Ang range you quote obviously only applies to the kind
of atoms we as macromolecular crystallographers are interested in,
i.e. mostly carbon with some nitrogen & oxygen, and also at the
'typical' data resolution and B factors that we observe. For hydrogen
it would obviously be a lot more, for heavier atoms a good deal less.
My point is that the positional uncertainty (I assume that by
'distance' you meant 'position' not 'bond length', otherwise we also
have to consider the effect of geometric restraints) is strongly
dependent on the atomic number and this indeed gives the clue as to
its origin. The prior information about _atomicity_ that we are
adding (via the scattering factors) exerts a powerful effect in
reducing the positional uncertainty well below the nominal resolution,
just as prior information in the form of geometric restraints reduces
the uncertainties in the bond lengths and angles.
David Moss & I (with Roman Laskowski) published something on the lines
of correlating positional precision with atomic number, B factor,
resolution and geometric restraints while I was at Birkbeck [Acta
Cryst. (1998). D54, 243-252], and I know others (Cruickshank, Blow,
Sheldrick) have done something similar before and since.
Cheers
-- Ian
On Thu, Dec 23, 2010 at 9:20 AM, Nicholas keep
<[log in to unmask]> wrote:
> We clearly have confidence in distance measurements in crystal structures of
> an order of magnitude better than the resolution ie 0.1-0.3 Angstroms, but
> can anyone point me to a more exact theory of distance accuracy compared to
> optical resolution, preferably one that would apply to microscopy as well.
> Have a Happy Christmas and see many of you at CCP4
> Nick
>
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