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RE: Phase ID draft report

Dear David:

Thanks for emphasising the distinction being made between the number
of atoms with given Wyckoff site symmetry and the multiplicity of the site
itself in your proposal. It would indeed be well worth the effort of making
distinction quite clear.

It is always disappointing when committee members with appropriate expertise
find themselves too busy (or whatever) to contribute appropriately and I had
hoped this working group might have had fewer than usual such members. I
would gladly be surprised by many last minute sets of contributions!


-----Original Message-----
From: phase-identifiers-bounces@iucr.org
[mailto:phase-identifiers-bounces@iucr.org]On Behalf Of David Brown
Sent: Thursday, May 06, 2004 12:11 PM
To: A Working Group of the IUCr Commission on Crystallographic
Subject: Re: Phase ID draft report

Dear Sidney,

Thanks for your careful reading of the proposed report on the phase
identifier. I will incorporate your comments in the next draft. I only
want to comment here on one item to clear up a misunderstanding.

> 7. In “6.3 Layer 7. Wyckoff Sequence”
> It would be more appropriate here to use the same Wyckoff site order
> as used in ITA, i.e. number, followed by letter. It would also be
> helpful if the example contained at least two sites with multiplicity
> other than 1, so that it becomes, for example, ‘1a 2c 3f 6g’ or
> ‘a2c3f6g’, to avoid possible confusion with the dropped ‘1’ (which I
> agree should be dropped).
There is a confusion between the multiplicty of the Wyckoff position and
the number of symmetry-independent atoms that occupy the position. In
the example given (adi6), a and d presumably have different
multiplicities, say 1 and 4, but these are not shown. i is likely to be
a general position with, say, multiplicity 8. The multiplicities are not
shown as they can be found in ITA since the space group number is known,
but the total number of atoms in the cell in this example would be 1x1 +
1x4 + 6x8 = 53. This point clearly needs to be made clearer and I will
do this in the next version.


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