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March 2002 issue
Cover illustration: Electrostatic equilibrium of simultaneous Coulomb attraction and core repulsion for an ensemble 
; the conjoint centroids, shown in yellow, characterize force-free positions of the fragment inside the unit cell. For comparison, the pure Coulomb equilibrium is shown in the background. See Mädler, Behrends & Knorr [Acta Cryst. (2001). A57, 20-23].
research papers
A method of systematically enumerating homogeneous (i.e. symmetry-related) packings of equal cylinders is developed.
A time-resolved stroboscopic study of the photo-induced triplet excited state of the [Pt2(pop)4]4− ion (pop = pyrophosphate), performed at He temperatures with synchrotron radiation, is reported.
Calculations using the Frenkel–Kontorova and double-chain models lead to new results explaining the experimental diffraction pattern of K-hollandite. The use of a deformable host chain and a Lennard-Jones-like interchain interaction gives the best results. It is concluded that the guest as well as the host are deformed and that the system is pinned. The inclusion of Debye–Waller factors as fitting parameters is also discussed, with the conclusion they should not be used in the case of the double-chain model.
A new type of X-ray phase-retarder system that can compensate for aberrations (phase-shift inhomogeneity) not only due to angular divergence of incident X-rays (off-axis aberration) but also due to energy spread (chromatic aberration) has been developed. The principle of this new X-ray optical system, which consists of four transmission-type diamond X-ray phase retarders whose planes of incidence are inclined with respect to the horizontal plane by 45, 135, 225 and 315°, is described together with experimental results revealing its advantage over a single phase retarder.
The coincidence site lattice theory of triple junctions is applied to grain-boundary dislocation interactions.
The use of different atomic and molecular fragments is analyzed for reproducing the electrostatic potential of proteins in order to enhance structure refinement in electron crystallography.
It is shown how Voronoi polyhedra for C and H atoms in polynuclear aromatic hydrocarbons display characteristic differences that allow perfect atom-type prediction from knowledge of the atomic coordinates alone.
A general method for obtaining periodic average structures of a decagonal quasicrystal is presented. New insights into the concept of periodic average structure are revealed. The method, based on reciprocal space, allows an easy comparison between quasicrystals and related periodic phases.
For imperfect crystals, a discontinuous physical model is suggested in relation to γ-ray diffraction. On this basis, a close dependence of the extrapolation procedure on the experimental measurements is demonstrated.
A new approach for the modelling of diffraction patterns without using analytical profile functions is described and tested on ball milled f.c.c. Ni powder samples. The proposed whole powder pattern modelling (WPPM) procedure allows a one-step refinement of the most relevant microstructure parameters (including those of domain size and lattice defects) by a direct modelling of the experimental pattern.
Triplet invariants of the native protein are estimated when diffraction amplitudes of the derivative and of the heavy-atom structure are available.
addenda and errata
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letters to the editor
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The definition of quasicrystals suggested by the International Union of Crystallography Ad Interim Commission on Aperiodic Crystals is recommended.
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Responses are made to K. H. Kuo's three comments in the previous paper concerning quasicrystalline phases and the examples of quasicrystalline phases in the recommendations presented earlier.
