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AW: further proposal
- To: Multiple recipients of list <phase-identifiers@iucr.org>
- Subject: AW: further proposal
- From: "Dr. Michael Berndt" <berndt@crystalimpact.com>
- Date: Tue, 3 Dec 2002 14:06:51 GMT
Dear David, thank you for introducing me into this group. We (Crystal Impact) are currently developing a new database management system called 'Voyager', which serves as universal interface between existing crystallographic databases (ICSD, PDF, Pauling File, etc.). One of the most challenging tasks is the development of algorithms for the automatic assignment of database entries (describing one structure/compound from one bibliographic reference) to distinct phases. We found that if the structure is fully determined, it is always possible to assign entries to unique phases. But the resulting phase identifier has - at least in some cases - an 'external' component, which cannot be derived from properties 'internal' to the material. Our approach is only applicable on fully determined crystal structures which are of course only a part of the target. It starts with the formula as described as item 7 in your proposal. As it is completely redundant and more informative than the chemical system, I think we don't need the chemical system. The second component is the Wyckoff sequence, consisting of the space group number and a sorted sequence of the numbers and Wyckoff letters of all occupied atom sites, e.g. '174, k3 j3 e d b'. This descriptor is a very convenient parameter for the classification of structure types and permits a much finer subdivision of structure types than the Pearson symbol. A more or less complex standardization procedure is necessary to get a unique Wyckoff sequence, but this can be done by computer programs. By the way, it is true that wrong assigments of space groups appears frequently, but we should not suppose that in general. There already exists software which helps to find those errors and it should be applied systematically. In rare cases we find compounds with the same chemical composition and the same Wyckoff sequence but nevertheless describing different modifications (Example: ICSD entries 24674 and 27116; both H7 N O6; Wyckoff sequence: 19, a7). In these cases we need a third, empirical component for the phase identifier, e.g. 'H7 N O6, 19, a7, I' and H7 N O6, 19, a7, II'. The resulting identifier allows the selection of entries belonging to the same phase and this is the key to use databases like the ICSD as 'phase-oriented databases'. So as a conclusion I propose to consider the Wyckoff sequence as additional parameter for the phase identifier - maybe as a replacement for the Pearson symbol, since the informations are redundant. We are currently at the implementation and testing stage for the automatic phase assignment. It will be interesting to see, how much it can reproduce e.g. the assignments in Pauling File which are made by editors. I will report on the results when they are available... Best regards Michael ------------------------------------------ Dr. Michael BERNDT Crystal Impact GbR Postfach 1251 D-53002 Bonn Germany Phone: +49 - 228 - 9813643 Fax: +49 - 228 - 9813644 Email: berndt@crystalimpact.com http://www.crystalimpact.com ------------------------------------------
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