The program Mercury, developed by the Cambridge Crystallographic Data Centre, is designed primarily as a crystal structure visualization tool. A new module of functionality has been produced, called the Materials Module, which allows highly customizable searching of structural databases for intermolecular interaction motifs and packing patterns. This new module also includes the ability to perform packing similarity calculations between structures containing the same compound. In addition to the Materials Module, a range of further enhancements to Mercury has been added in this latest release, including void visualization and links to ConQuest, Mogul and IsoStar.

A method is presented for comparing crystal structures to identify similarity in molecular packing environments. The relative position and orientation of molecules is captured using interatomic distances, which provide a representation of structure that avoids the use of space-group and cell information. The method can be used to determine whether two crystal structures are the same to within specified tolerances and can also provide a measure of similarity for structures that do not match exactly, but have structural features in common. Example applications are presented that include the identification of an experimentally observed crystal structure from a list of predicted structures and the process of clustering a list of predicted structures to remove duplicates. Examples are also presented to demonstrate partial matching. Such searches were performed to analyse the results of the third blind test for crystal structure prediction, to identify the frequency of occurrence of a characteristic layer and a characteristic hydrogen-bonded chain.

The probability of organic compounds crystallising as hydrates increases with increasing number of polar chemical groups in the molecule. The extended patterns of H-bonding involving chemical groups and water molecules have been studied and classified. The most frequent ring, chain, tape and layer patterns displayed between the water molecules alone in organic molecular crystals are also predominant patterns in the larger H-bond network when other donor/acceptors are included.

Lattice energy searches for theoretical low-energy crystal forms are presented for 50 small organic molecules, and we compare the experimentally observed crystal forms to these lists of hypothetical polymorphs. For each known crystal, the relative stability is calculated with respect to the global minimum energy structure, and we determine the number of unobserved structures lower in energy than the experimental form. The distributions of these relative energies and their rankings in the predicted lists are used to determine the efficacy of lattice energy minimization in crystal structure prediction. Although a simple form for the interaction energies has been used, the calculations produce almost a third of the known crystals as the global minimum in energy, and approximately a half of the known structures are within 1 kJ/mol of the global minimum. Molecules with no hydrogen-bonding capacity are most likely to be found close to the global minimum in lattice energy, while increasing the number of possible hydrogen-bond donor-acceptor combinations leads to less reliable predictions.

How does one find out whether an interesting water aggregate observed in an organic crystal structure is novel? The first place to look is in the Cambridge Structural Database (CSD). The data are there, but analysis is difficult and it is not routinely done. The result? Many of the water chains, rings, sheets, and clusters reported as novel in the recent literature are actually redundant with published structures in the CSD.

Statistical models to predict the number of hydrogen bonds that might be formed by any donor or acceptor atom in a crystal structure have been derived using organic structures in the Cambridge Structural Database. This hydrogen-bond coordination behaviour has been uniquely defined for more than 70 unique atom types, and has led to the development of a methodology to construct hypothetical hydrogen-bond arrangements. Comparing the constructed hydrogen-bond arrangements with known crystal structures shows promise in the assessment of structural stability, and some initial examples of industrially relevant polymorphs, co-crystals and hydrates are described.

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To investigate the merits of crystal structure prediction using ab initio computational techniques, we have used density functional (DFT) methods to investigate the relative stabilities of the four known crystalline phases of glycine and also a range of alternative putative crystal structures of the zwitterion. Energy differences are calculated using a range of exchange-correlation functionals, and it is found that the calculated relative stability of the phases is sensitive to the choice of functional. Energy differences are found to be on the order of a few tenths of a kilocalorie per mole with little separation in energy found between observed and putative structures. This result is similar to that typically obtained from force field calculations and confirms the difficulty of the task of predicting the structure of molecular crystals. Optimization of structures, including optimization of unit cells, highlights the limitation of DFT in describing the long-range dispersion interaction. Use of the local density approximation (LDA) is shown to over-bind crystals, whereas use of gradient-corrected functionals severely underbinds crystals. Calculated structural energy differences are presented, which show that, for the case of the LDA, the four observed glycine polymorphs receive a lower energy than all putative glycine structures considered.