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Re: New version of CIF Core dictionary (2.4)



Title:
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
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fn:I.David Brown
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org:McMaster University;Brockhouse Institute for Materials Research
adr:;;King St. W;Hamilton;Ontario;L8S 4M1;Canada
email;internet:idbrown@mcmaster.ca
title:Professor Emeritus
tel;work:+905 525 9140 x 24710
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