IUCr activities

CIFNEWS-9: New CIF dictionaries and International Tables Volume G

[Brown] I.D. Brown

The publication of Volume G of International Tables for Crystallography, scheduled for 2004, will provide crystallographers with a comprehensive description of the CIF project ranging from an account of its history and philosophy to details of all the approved CIF dictionaries. It will answer all the questions you ever had about CIF and even questions you probably never thought of. Each of the CIF dictionaries will be accompanied by an article outlining the rationale for the choice of the items that are included and how these are organized, making this volume the complete reference source on the topic.

To ensure that this volume is as current as possible, COMCIFS has been working hard to update the suite of CIF dictionaries before the copy deadline. Consequently this past summer has seen the approval of a new dictionary and the revision of two others. These dictionaries can be viewed on the IUCr web site, www.iucr.org, in a variety of formats. The canonical version of each dictionary is an ASCII text file using the STAR syntax (the same syntax as is used in CIF). This version has the complete text and is the one used as input to CIF applications, such as the recently released editor and browser CIFEDIT [Toby (2003), J. Appl. Cryst. 36, 1288-1289]. The other formats in which the dictionaries are presented, such as html and pdf, are designed to make the dictionaries easy to read.

The cif_core dictionary will be familiar to anyone working in small-cell crystallography (inorganic and small molecule compounds). It is the preferred format for publication of structure determinations, and it is increasingly being adopted for the interchange of crystallographic information between applications. Its widespread use over the past 10 years has revealed areas where improvements and additions are needed. As a result the cif_core dictionary is currently undergoing a major review which will continue well into 2004. The recently approved version 2.3 contains some of the less controversial proposals. These include the replacement of the _symmetry items by a more systematically defined set of _space_group items from the newly adopted symmetry CIF dictionary. Other changes include the addition of items to permit the Cambridge Crystallographic Data Centre to provide a CIF output from the Cambridge Structural Database. Still to come are items that will describe crystal twinning and the chemistry of molecules. Further information is available at www.iucr.org/iucrtop/cif/cif_core/index.html and the dictionary itself can be downloaded from ftp://ftp.iucr.org/pub/cif_core_2.3.dic.

ImgCIF is used for recording images, specifically the two-dimensional images of diffraction patterns that are now being routinely measured. This dictionary defines the items used in the ASCII version of an imgCIF as well as its isomorphous Crystallographic Binary File (CBF). The primary purpose of the cif_img dictionary is to extend the cif_mm dictionary to allow CIFs to record synchrotron diffraction images for structural biology. Writing applications using this dictionary revealed the need for a number of technical changes and these have been included in the recently approved version 1.3.1. Further information can be found at http://www.iucr.org/iucr-top/cif/imgcif/index.html and the canonical ASCII version of the dictionary is available from ftp://ftp.iucr.org/pub/cif_img_1.3.1.dic.

A new dictionary, cif_rho_1.0, has also been added to the suite of approved dictionaries. It is designed to archive electron density (ρ) measurements using the multipole expansion model proposed by Stewart [(1973), J. Chem. Phys. 58, 1668] and developed by Hansen and Coppens [(1978), Acta Cryst. A34, 909-921]. It is a relatively small dictionary designed as a supplement to the cif_core dictionary. Further information is on the rhoCIF home page www.iucr.org/iucr-top/cif/rho/index.html. The canonical ASCII version of the dictionary can be downloaded from ftp: //ftp.iucr.org/pub/cif_rho_1.0.dic.

David Brown, Chair, COMCIFS