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Re: Masking

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David J. Schuller wrote:

> there is no such thing as maskless averaging.

I do not agree with this statement. A properly conceived density averaging
program will offer this possibility.
For improper NCS a mask is required to get the same results than those
obtained with proper NCS and no mask, see below.

to define the situation properly, take an example eg for a 2-fold
improper NCS:
the operator to go from zone 1 to zone 2 is different than that
used to go from zone 2 to zone 1.

For proper 2-fold NCS, the same operator relates (zone 1 to zone 2) and
(zone 2 to zone 1).

So, for proper NCS (e.g. 2-fold) one can give an e.d. map centered on
particle on which averaging is to be carried out and "do it" without a
mask. The density for both zones 1 and 2, where the NCS applies, will
improve by the averaging operation, and the density outside will

For improper symmetry, with a map centered as above, no mask: if one uses
the operator relating (zone 1 to zone 2) then the density for zone 1 will
improve whereas all the rest, including zone 2, will deteriorate. And the
opposite will take place if one uses the operator relating (zone 2 to
zone 1).

The same considerations also apply for the calculation of real space
correlation coefficients.

So "maskless averaging" can be used to derive real space masks.
But this operation heavily relies on the use of the correct
NCS operators.

It is just that having improper NCS means more work for the
crystallographer than having proper NCS.


s-mail: F.M.D. Vellieux (B.Sc., Ph.D.)
        Institut de Biologie Structurale J.-P. Ebel CEA CNRS
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