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Anisotropic displacement parameters (ADPs) are compared for H atoms estimated using three recently described procedures, both among themselves and with neutron diffraction results. The results convincingly demonstrate that all methods are capable of giving excellent results for several benchmark systems and identify systematic discrepancies for several atom types. A revised and extended library of internal H-atom mean-square displacements is presented for use with Madsen's SHADE web server [J. Appl. Cryst. (2006), 39, 757–758; http://shade.ki.ku.dk], and the improvement over the original SHADE results is substantial, suggesting that this is now the most readily and widely applicable of the three approximate procedures. Using this new library – SHADE2 – it is shown that, in line with expectations, a segmented rigid-body description of the heavy atoms yields only a small improvement in the agreement with neutron results. The SHADE2 library, now incorporated in the SHADE web server, is recommended as a routine procedure for deriving estimates of H-atom ADPs suitable for use in charge-density studies on molecular crystals, and its widespread use should reveal remaining deficiencies and perhaps overcome the inherent bias in the majority of such studies.

Extensive and precise X-ray diffraction data for xylitol have been used to test different approaches to estimate nuclear parameters for H atoms in charge-density studies. The parameters from a neutron diffraction study of the same compound were taken as a reference. The resulting static charge densities obtained for the different approaches based on a multipole model were subjected to a topological analysis. The comparative analysis led to the following results. The procedure of extending the X-H bond to match bond lengths from neutron diffraction studies provides the best agreement with the neutron positional parameters. An isotropic model for the atomic displacements of H atoms is highly unsatisfactory and leads to significant deviations for the properties of the bond critical points including those that only involve non-H atoms. Anisotropic displacement parameters for H atoms can be derived from the X-ray data that are in agreement with the values from the neutron study, and the resulting charge-density models are in good agreement with the reference model. The anisotropic displacement parameters for H atoms are derived from the X-ray data as a sum of the external (rigid-body) and internal vibrations. The external vibrations are obtained from a TLS analysis of the ADPs of the non-H atoms and the internal vibrations from analysis of neutron diffraction studies of related compounds. The results from the analysis of positional and thermal parameters were combined to devise a 'best anisotropic' model, which was employed for three other systems where X-ray and neutron data were available. The results from the topological analysis of these systems confirm the success of the 'best anisotropic' model in providing parameters for the H atoms that give charge densities in agreement with the reference models based on H-atom parameters derived from neutron diffraction.

It is well known that the use of synchrotron radiation has revolutionised structural biology and is responsible for the exponential growth of known macromolecular structures in the Protein Data Bank. So onemaywonder if continued progress is possible. The recent developments of beamlines for macromolecular crystallography at the ESRF have had the purpose to make them highly automated. This has enabled the users to screen a multiplum of samples to find the best suited crystal for data collection. The use of crystallization robots tends to yield smaller crystals, which creates an increasing demand for beamlines with small beams to be used for the diffraction experiments. These developmentsmake it possible to study larger andmore complex biological systems, bringing the structural biology closer to the studies of soft condensed matter. We can also observe an increasing interest to complement the diffraction studies with complementarymethods like small angle scattering and imaging. The use of nanometer sized X-ray beams is also in demand for investigations of chemical systems. The use of nanosized beams opens possibilities for an unprecedented spatial resolution that open new scientific possibilities inmaterials science, soft matter chemistry and the characterization of complex chemical systems. The possibility of conducting the experiments at the synchrotron on samples under different thermodynamic conditions has stimulated an avenue of studies of chemical systems under extreme conditions. There is a similar interest in employing the time structure of synchrotron radiation in time resolved studies. The trends in the application of synchrotron radiation in chemistry and biology will be described and illustrated by recent scientific achievements [1].

Interactions between chiral molecules play a fundamental role for the most biological and many chemical processes. Due to the complexity of the interactions that range from normal hydrogen bonds to weaker interactions like C-H—O hydrogen bonds and stacking, it can be difficult to relate overall thermodynamic properties to the nature of the intermolecular interactions. The topological analysis developed by R.W. Bader for theoretical electron densities has proven to be a powerful tool to analyse intermolecular interactions in crystals based on an analysis of experimental electron densities obtained from accurate high resolution X-ray diffraction data. Our recent investigations of the molecular crystal naphthalene revealed that the topological analysis could reveal and characterize unexpected interatomic interactions [1]. The two pentose epimers xylitol and ribitol differ in the physico-chemical properties, the acentric xylitol being the higher melting and the less dense than ribitol. All possible hydrogen bond donors participate in the hydrogen bond interactions in both compounds, and the topological analysis confirms the energetic similarity of these interactions. The investigations suggest that the origin of the differences in physico-chemical properties is related to differences in the weaker intermolecular interactions and in the entropy. Crystallization of a racemate may lead to racemic compound composed of equal amounts of the two enantiomers or to a spontaneous resolution of crystals that are formed by the pure enantiomers, and the outcome of the process depends on differences in enthalpy and entropy. We have investigated the intermolecular interactions in closely related crystal structures, e.g. a racemic para-fluoromandelic acid and its corresponding enantiomer. The racemic para-fluoromandelic acid showing the unusual behaviour of being the higher melting. An earlier investigation showed that the two forms have an identical hydrogen bond pattern. This was confirmed by the topological analyses based on accurate X-ray diffraction data. Though additional features were revealed from the topological analysis it also showed the significance of supplementing the investigations with theoretical calculations.