issue contents

ISSN: 2053-2733

November 2009 issue

Highlighted illustration

Cover illustration: Electron-density isosurface of silicon coloured with electrostatic potential (red for lowest and blue for highest), together with a convergent-beam electron-diffraction (CBED) pattern of silicon. The red regions between neighbouring atoms indicate that the electrostatic potential is lowered due to the excess (bonding) electrons. The electrostatic potential and electron density were determined using a CBED structure-refinement method [Ogata et al. (2008), Acta Cryst. A64, 587-597]. The figure was drawn using VESTA [Momma & Izumi (2008), J. Appl. Cryst. 41, 653-658].

research papers

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A method of structure determination based on an equal-atom point model is described. The method avoids the phase problem, offers high resolution, works with sparse experimental data and provides all possible (quasi-)homometric solutions.

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An open-ended classification scheme for crystal structures based on Wyckoff sets and affine normalizer groups is proposed.

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The classification of diffraction spectra of aperiodic crystals based on multiresolution analysis is described.

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On the basis of high-resolution single-crystal X-ray diffraction data for paracetamol, different approaches (including those based on pseudoatom databases) to modeling of the static experimental electron density in the presence of a dynamically disordered molecular fragment were tested. The electrostatic properties obtained were compared with the results of theoretical single-point periodic calculations.

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The mineral kettnerite, CaBi(OFCO3), has an order–disorder (OD) structure with a quadratic net built up by more than one kind of layer. Two maximum-degree-of-order (MDO) (1O, 2O) and one non-MDO (2Q) polytypes are derived by stacking of regularly alternating polar OD packets instead of layers. The stacking rule allows the existence of out-of-step, twin and upside-down domains.

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The method of joint probability distribution functions has been applied to molecular replacement techniques.

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The success rate of the direct-methods sign-unconstrained S-FFT algorithm has been improved for density functions with only light scatterers. In addition, a closely related but simpler phase-refinement function is introduced, which can also treat positive and negative density peaks.

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An analysis of certain types of multiplicative congruential generators – otherwise known for their application to the sequential generation of pseudo-random numbers – reveals their relation to lattice–sublattice transformations and the coordinate description of crystal structures.

book reviews

international union of crystallography

Acta Cryst. (2009). A65, 546
doi: 10.1107/S0108767309042445

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