RE: A managed phase-out of DDL1 dictionaries
- To: "Discussion list of the IUCr Committee for the Maintenance of the CIFStandard (COMCIFS)" <firstname.lastname@example.org>
- Subject: RE: A managed phase-out of DDL1 dictionaries
- From: "Brown, David" <email@example.com>
- Date: Fri, 5 May 2017 19:31:54 +0000
- Accept-Language: en-CA, en-US
- In-Reply-To: <CAM+dB2fxfyrV5edSYH3jd3xPcHnCXYVXM-Mi2uSi8n-Sq+bXeQ@mail.gmail.com>
- References: <CAM+dB2fy=0ASMnwF6Rze++qdQ13-ab=dTwUO7SV4NsLgUcFOvg@mail.gmail.com><D9E3F175030713438B5B5883ABA5812B5AAEEAE5@mail03.ccdc.cam.ac.uk><CAM+dB2ddCvQWJZQB8Q-A+qo=Zf0-sHv_AROJJikiXe+=8Ew3jw@mail.gmail.com><D9E3F175030713438B5B5883ABA5812B75129400@mail03.ccdc.cam.ac.uk>,<CAM+dB2fxfyrV5edSYH3jd3xPcHnCXYVXM-Mi2uSi8n-Sq+bXeQ@mail.gmail.com>
Perhaps an explanation why "space_group_name_H-M_alt" was defined as loopable is in order. When we put together the symmetry dictionary we were accommodating two schools with very different interest in symmetry. For most crystallographers symmetry is a convenient shortcut for defining all the atomic positions in the unit cell when symmetry is present, but there is another school that is interested in the theory of symmetry as a rigorous mathematical subject, and those of us who tend to be a little sloppy in our use of symmetry (see _symmetry_cell_setting as an egregious example) do well to pay attention to what the theorists tell us. They needed a unique reference setting to be defined and this is given in "_space_group_H-M_ref". The choice of reference setting is necessarily arbitrarily and is defined in its enumeration list.
Structure solvers choose the setting that is most natural to the structure they are describing. They are the people most likely to use CIF, but we wanted to make sure that the theorists could, for example, list all the settings for a given space group in the same CIF, as explained in the last sentence of section 3.8.2 in the original version of ITG. We therefore included "space_group_name_H-M_alt" as an opportunity for one or more non-reference settings to be given. This would require "space_group_name_H-M_alt" to be looped. It is hardly surprising that one does not find it looped in the Acta Cryst. database of structures, but I would be sorry to see its potential to describe symmetry in a more general and theoretical way excluded from CIF2. Its inclusion as a loopable item was deliberate and should be respected.
I. David Brown
Department of Physics and Astronomy
Hamilton, Ontario, Canada
From: comcifs [firstname.lastname@example.org] on behalf of James Hester [email@example.com]
Sent: May 3, 2017 01:02
To: Discussion list of the IUCr Committee for the Maintenance of the CIF Standard (COMCIFS)
Subject: Re: A managed phase-out of DDL1 dictionaries
Dear Matthew,We definitely need to take into account the significant investment of various groups in software based on the DDL1 framework. COMCIFS have guaranteed that datanames defined in DDL1 dictionaries retain the same meaning when appearing in DDLm dictionaries, so software that works with DDL1 datanames can function unchanged. If changes to the legacy dictionaries actually required you to spend any time rewriting software, then they are highly unlikely to be acceptable. All changes will be drafted and published first in the Github repository (https://github.com/COMCIFS/DDL1-legacy-dictionaries), and I'd encourage you to monitor that and participate in discussions, particularly if any proposed changes would have a negative impact on your activities.
A typical example of the types of changes being entertained would be "space_group_name_H-M_alt" being defined as loopable in the DDL1 core dictionary but strictly unlooped in the DDLm dictionary (discussed at https://github.com/COMCIFS/cif_core/issues/20). Examination of public CIF corpuses (e.g. IUCr, COD) suggest that nobody has ever looped this dataname, therefore aligning it with the more rigorous DDLm definition would seem feasible.
On 3 May 2017 at 01:24, Matthew Towler <firstname.lastname@example.org> wrote:
T +61 (02) 9717 9907
F +61 (02) 9717 3145
M +61 (04) 0249 4148
Reply to: [list | sender only]
- Re: A managed phase-out of DDL1 dictionaries (James Hester)