issue contents

ISSN: 2053-2733

September 2006 issue

Highlighted illustration

Cover illustration: Determination of a one-dimensional structure by the charge flipping algorithm. Bottom curves: grey is the initial electron density obtained with a random choice of structure-factor phases; white is the result of charge flipping, i.e. the sign of the density below a threshold is reversed. Top curves: electron density after convergence (blue) and its flipped counterpart (red). Only the three atomic regions correspond to the true structure, small oscillations do not. See Oszlányi & Süto [Acta Cryst. (2004), A60, 134-141; (2005), A61, 147-152].

research papers

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The surface structures of viral capsids are described in terms of polyhedra with icosahedral symmetry and rational indices. It is shown that the Caspar–Klug and the Twarock approaches can be embedded into this framework. The crystallographic scaling transformations are worked out explicitly and are used to derive the icosahedral forms for some viruses, including the Caspar–Klug and Twarock cases.

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A more general probability density function of the triple-phase sums capable of predicting negative values is derived with the maximum-likelihood method. It is also demonstrated that maximization of the origin-free modulus sum function yields the maximum-likelihood estimates of the phases.

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An algorithm for the derivation of twin laws in non-merohedric twins and for the analysis of the hybrid twins is presented. A Fortran program is available.

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28 nets, and their natural tilings, with two kinds of vertex and one kind of edge are described.

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A method is proposed for hierarchical enumeration and topological comparative analysis of all crystallochemically significant representations for compounds of any composition and chemical nature. The advantages of the method are shown by the analysis of typical inorganic compounds and a molecular packing.

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Electron-density determination of L-histidinium dihydrogen orthophosphate orthophosphoric acid from X-ray and neutron diffraction data. The effect of the refinement strategy on the topological properties of ρ(r) in the hydrogen bonds and the use of the transferability principle to obtain accurate thermal parameters are analysed.

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For the 13 trivariant lattice complexes of the trigonal space groups, all types of homogeneous sphere packings, interpenetrating sphere packings and layers are derived. Examples of related crystal structures are given.

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Accurate, simple and efficient formulae for calculation of the electrostatic potential, electric field and electric field gradient from the aspherical pseudoatom model of electron density are presented. The expressions are applied to the determination of the nuclear quadrupole moment of the Fe atom in Fe(CO)5.

short communications

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This short communication discusses the connections between sublattice counting methods and their relation to Gaussian binomials.

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A recurrence relation is shown to exist between O-lattices that induces a partition of the angular space into a finite number of equivalence classes. This establishes a much sought after general classification for rotation-related interfaces derived from first principles.
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