issue contents

ISSN: 2053-2733

May 2002 issue

Highlighted illustration

Cover illustration: Electrostatic equilibrium of simultaneous Coulomb attraction and core repulsion for an ensemble {\cal F}=(X_1^-,X_2^-); 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

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A class of functions representing localized atomic form information is introduced and phase refinement and automated map interpretation aspects are investigated. A simple iterative phase refinement procedure based on atomic form constraints is applied to three small metalloproteins, with overall improvements of 20 to 40° in mean phase error and 0.2 in map correlation.

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The anisotropic Porod law is extended to particulate samples where particles have non-convex shapes with possible planar facets.

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A new constrained wavefunction model that utilizes experimental X-ray structure factors has been used to produce a charge density distribution for crystalline α-oxalic acid dihydrate. Topological and integrated atomic properties have been derived from this electron density, including estimates of the experimental errors associated with these quantities, and the influence of both the basis sets and the choice of experimental data on these properties has been examined.

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The constrained wavefunction procedure has been applied to experimental data for NH3. Different molecular environments have been used to construct the theoretical crystal and integrated atomic properties have been derived. The accuracy of these properties and the error in the experimental σ(k) values has been estimated.

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The analytical theory for three-beam diffraction based on a Born series for the displacement field in a semi-infinite crystal is reexamined. Results are obtained in agreement with the Takagi–Taupin formalism.

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A new optimization approach, energy landscape paving, is employed for finding the crystal structure of organic molecules. The efficiency of this method is compared with simulated annealing.

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The Schrödinger equation as the basis of a novel solution of the phase problem. Theoretical developments.

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Motivations to use likelihood-based residuals are discussed. The quadratic approximation of the likelihood function enables one to consider the likelihood-based refinement as a kind of conventional least-squares refinement and to study the differences between these criteria.

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The phase problem may be reduced to the solution of a system of linear inequalities in binary unknowns. Integer programming can be used for low-resolution ab initio phasing.

short communications

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An expression for cylindrically averaged intensity diffracted by a partially occupied helix (i.e. by a set of identical molecules bound to some points of a discrete helix) is derived. The result is compared with previous studies and its application to muscle diffraction is discussed.

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The most symmetrical C50 to C70 fullerenes with minimum numbers of pairs of adjacent pentagonal facets are considered in this paper.

addenda and errata

letters to the editor

Acta Cryst. (2002). A58, 301
doi: 10.1107/S0108767302001368
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An error is corrected in the tabulation of enantiomorphic space-group types given in the book Crystallographic Groups of Four-Dimensional Space by Brown et al. (1978).

international union of crystallography

Acta Cryst. (2002). A58, 302
doi: 10.1107/S0108767302005573

book reviews

Acta Cryst. (2002). A58, 304
doi: 10.1107/S0108767302003288
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