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- To: "Discussion list of the IUCr Committee for the Maintenance of the CIF Standard (COMCIFS)" <firstname.lastname@example.org>
- Subject: Re: Variants
- From: David Brown <email@example.com>
- Date: Thu, 26 Nov 2009 11:52:51 -0500
- In-Reply-To: <firstname.lastname@example.org>
- References: <email@example.com> <alpine.BSF.firstname.lastname@example.org><email@example.com>
The idea of including variants is fascinating, but the implications
for CIF are fearsome and we need to tread carefully. At the very least
this is not the time to launch into something of this size,
particularly when we can't even decide what a reverse solicus means. I
have a couple of observations to make to put the issue into focus.|
1. The proposal I made is for the coreCIF DDL1 dictionary and I would have to be convinced that we can easily graft 'variants' into DDL1. This would require giving the implications of introducting variants a great deal more thought and my proposal would no longer be fast-track. The wavelength items are already looped in the DDL1 version of coreCIF, since powder patterns often have to deal with the prsence of multiple wavelengths. Since there is no dREL in DDL1, it is up to the application to decide what to do if more than one wavelength is given.
2. If we want to introduce variants, CIF2 would be the place to do so. With dREL, CIF2 has a more dynamic structure which better allows for a CIF to evolve as a structure is determined. CIF1.1 was conceived as a static representation of the results of a crystallographic experiment.
3. If variants are being introduced into CBF, are they also allowed in imgCIF? This would need to come before COMCIFS for approval, but if it is approved by COMCIFS it would automatically become part of the DDL2 protocol and so presumably could be merged into other DDL2 dictionaries.
4. However, its spread can be limited because it can only be used if items such as:
are defined in the dictionary. Thus if Herbert wants to introduce this into imgCIF, it can easily be confined to that dictionary, until such time as COMCIFS is ready to define the necessary datanames to implement it in other parts of the system.
5. Variants have important implications for dREL which currently has no mechanism for selecting which value of _diffrn_radiation_wavelength to use. There are other aspects of dREL that still need to be clarified, such as are multiple definitions of the same item allowed and if so which will be executed. Let's not get into that until we know what "\" means.
The implications of variants are many and interesting, but they repesent a change in the philosphy of CIF. I am not against variants, the idea is quite exciting, but we need to give careful thought about where we see CIF going over the next decades. Is a CIF a fixed archive, a place where we entomb a fact of nature, or is it an evolving document that carries it own history along with it? Now is not the time to get distracted by such far reaching discussions, we just need to keep the options open.
James Hester wrote:
I include David's original message below for reference.
begin:vcard fn:I.David Brown n:Brown;I.David org:McMaster University;Brockhouse Institute for Materials Research adr:;;King St. W;Hamilton;Ontario;L8S 4M1;Canada email;internet:firstname.lastname@example.org title:Professor Emeritus tel;work:+905 525 9140 x 24710 tel;fax:+905 521 2773 version:2.1 end:vcard
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