New version of CIF Core dictionary (2.4)

[David Brown]

Dear Ralf,

Thank you for returning your approval of the proposed changes to the 
core CIF along with some suggestions for minor changes.  At least I 
assume your email indicated your formal approval of the changes apart 
from those noted below, most of which are easily fixed.


Ralf W. Grosse-Kunstleve wrote:

>==============================================================================
>data_distributed_density_symmetry_multiplicity
>    _enumeration_range            1:192
>==============================================================================
>
>The upper limit is rather arbitrary. It is true that the maximum
>multiplicity in the Int. Tab. is 192, but when working with
>group-subgroup relations (e.g. to analyze twinning) there could be
>unusual situations which require settings with larger multiplicities.
>I propose that the upper limit is removed.
>
This is a good point which we can accommodate.

>
>==============================================================================
>data_distributed_density_symmetry_multiplicity
>    _definition
>                                                   If the ring is at some
>               arbitrary angle to the mirror plane, the mirror generates
>               a second ring and both rings should be described independently.
>==============================================================================
>
>This seems inconsistent with the general approach of only providing
>non-redundant atomic data (asymmetric unit). In my working experience,
>having to deal with explicit redundancy is usually cumbersome and
>prone to introduce ambiguities and errors. I'd very much prefer if
>the core dictionary mandated a non-redundant representation.
>
This is a trickier point and your proposed correction is more than 
editorial.  It will require referral back to the Dictionary Maintenance 
Group.

>==============================================================================
>data_distributed_density_Uiso
>    _definition
>;              The factor exp(-Ux^-2^) is applied to all parts of the
>==============================================================================
>                               ^^^^
>                               2 pi ^2^ ?
>
>The formula for the Debye-Waller factor (atomic Uiso) is
>  exp(-2 pi^2^ U d^-2^)
>I find it misleading that the 2 pi^2^ is folded into
>_distributed_density_Uiso. For consistency, I'd change the
>definition to:
>  exp(-2 pi^2^ Ux^-2^)
>Another commonly used notation for the isotropic displacement parameter is
>"beta" (greek letter), with this definition:
>  exp(-beta d^-2^)
>This suggests the name _distributed_density_beta_iso.
>However, I'd prefer to keep _distributed_density_Uiso, but with
>a definition compatible with the atomic Uiso.
>
This is an oversight and will be changed.  The regular definition of U 
is intended.  Beta is certainly not an option.

>==============================================================================
>                    6.  _exptl_absorption_correction_T_max, *_T_min
>                                     ^^^^
>                                     wrong name
>
>                        exptl_transmission_factor_max and *_min
>==============================================================================
>                       ^
>                       missing underscore
>
Thanks for pointing these out.

Since you have indicated a serious reservation about the way in which 
symmetry is handled in the new distributed density category, we will 
withdraw this section and send it back for further discussion.

Best wishes

David
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://scripts.iucr.org/pipermail/comcifs/attachments/20080218/5ecfab0f/attachment.html 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: idbrown.vcf
Type: text/x-vcard
Size: 298 bytes
Desc: not available
Url : http://scripts.iucr.org/pipermail/comcifs/attachments/20080218/5ecfab0f/attachment.vcf