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The Cambridge Structural Database (CSD) now contains data for more than a quarter of a million small-molecule crystal structures. The information content of the CSD, together with methods for data acquisition, processing and validation, are summarized, with particular emphasis on the chemical information added by CSD editors. Nearly 80% of new structural data arrives electronically, mostly in CIF format, and the CCDC acts as the of®cial crystal structure data depository for 51 major journals. The CCDC now maintains both a CIF archive (more than 73 000 CIFs dating from 1996), as well as the distributed binary CSD archive; the availability of data in both archives is discussed. A statistical survey of the CSD is also presented and projections concerning future accession rates indicate that the CSD will contain at least 500 000 crystal structures by the year 2010.

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The Cambridge Crystallographic Data Centre (CCDC) was established in 1965 to record numerical, chemical and bibliographic data relating to published organic and metal-organic crystal structures. The Cambridge Structural Database (CSD) now stores data for nearly 700,000 structures and is a comprehensive and fully retrospective historical archive of small-molecule crystallography. Nearly 40,000 new structures are added each year. As X-ray crystallography celebrates its centenary as a subject, and the CCDC approaches its own 50th year, this article traces the origins of the CCDC as a publicly funded organization and its onward development into a self-financing charitable institution. Principally, however, we describe the growth of the CSD and its extensive associated software system, and summarize its impact and value as a basis for research in structural chemistry, materials science and the life sciences, including drug discovery and drug development. Finally, the article considers the CCDC's funding model in relation to open access and open data paradigms.

The nature of intermolecular interactions between carbon-bonded halogens (C-X, X ) F, Cl, Br, or I) and electronegative atoms (El ) N, O and S) has been analysed, focusing on the role of specific attractive forces and the anisotropic repulsive wall around halogen atoms. Searches of the Cambridge Structural Database show that electronegative atoms in various hybridization states clearly prefer to form contacts to Cl, Br, and I (but not F) in the direction of the extended C-X bond axis, at interatomic distances less than the sum of the van der Waals radii. Ab initio intermolecular perturbation theory calculations show that the attractive nature of the X‚‚‚El interaction is mainly due to electrostatic effects, but polarization, charge-transfer, and dispersion contributions all play an important role. The magnitude of the interaction for the chloro-cyanoacetylene dimer is about 10 kJ/mol, demonstrating the potential importance of these kinds of nonbonded interactions. The directionality of the interaction is explained by the anisotropic electron distribution around the halogen atom, causing a decreased repulsive wall and increased electrostatic attraction for electronegative atoms in the observed preferred position. In contrast, carbon-bonded hydrogens show no directionality in their contacts to the halogen atoms, because the angular dependence of the electrostatic energy is reversed and acts to counter rather then to reinforce the effect of the anisotropic repulsive wall.

The Cambridge crystallographic database comprises files of bibliographic, chemical connectivity and numeric structural data, for organics, organometallics and metal complexes studied by X-ray and neutron diffraction. The files, covering the literature from 1935 and maintained on a current basis, presently contain information on some 25 000 structural studies. Certain categories of information, particularly bibliographic, are disseminated in printed form via the Molecular Structures and Dimensions series. The full potential of the total database depends, however, on its response to specific user queries. The present paper describes the computer-based search and retrieval system, with special reference to question coding. The retrieved data may then be used for extensive and systematic geometric analysis, and for the visual display of crystal and molecular structures. The availability of the database is also discussed.as well as the establishment of abstracting, input and evaluation procedures. The utilization of sections of the database in the production of the reference-book series Molecular Structures and Dimensions (MSD) (Ken-nard &

The enCIFer program permits the location, reporting and correction of syntax and format violations in single-or multi-block crystallographic information ®les (CIFs). The program also permits the editing of existing individual or looped data items and the addition of new data in these categories, and provides dataentry wizards for the addition of two types of standard information for smallmolecule structural studies, namely publication data and chemical and physical property information. Facilities for the graphical visualization and manipulation of structure(s) in a CIF are also provided.