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The desirability and requirements for a specimen capable of testing the resolving power and other image characteristics of an electron microscope are discussed in detail. In this discussion, the underlying diffraction phenomena are particularly utilized. A partially graphitized carbon black is shown to satisfy the requirements extremely well and constitutes an easily prepared specimen for conducting tests of image quality in the molecular size range. The structure of the test object is known in detail with the result that readily interpretable phase contrast images are obtained. Micrographs illustrating the use of the 3.4 Å (002) spacing for magnification calibration, astigmatism and asymmetry check as well as resolving power are exhibited. The second order c-spacing of 1.7 Å is occasionally found in an image. The micrographs shown herein were taken with two different electron microscopes by different operators obtaining the same structural detail in the images. It is concluded that the carbon black test object offers the best possibilities for evaluating image performance of any specimen yet suggested.

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The PROCHECK suite of programs provides a detailed check on the stereochemistry of a protein structure. Its outputs comprise a number of plots in PostScript format and a comprehensive residue-by-residue listing. These give an assessment of the overall quality of the structure as compared with well refined structures of the same resolution and also highlight regions that may need further investigation. The PROCHECK programs are useful for assessing the quality not only of protein structures in the process of being solved but also of existing structures and of those being modelled on known structures.

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The MOLSCRIPT program produces plots of protein structures using several different kinds of representations. Schematic drawings, simple wire models, ball-and-stick models, CPK models and text labels can be mixed freely. The schematic drawings are shaded to improve the illusion of three dimensionality. A number of parameters affecting various aspects of the objects drawn can be changed by the user. The output from the program is in PostScript format.

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This paper describes the concepts of validation and the classes of checks that are carried out by the program PLATON as part of the IUCr checkCIF facility. PLATON validation can be run at any stage of the structure refinement, independent of the structure determination package used, and is recommended for use as a routine tool during or at least at the completion of every structure determination.

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A structure refinement method is described which does not use integrated neutron powder intensities, single or overlapping, but employs directly the profile intensities obtained from step-scanning measurements of the powder diagram. Nuclear as well as magnetic structures can be refined, the latter only when their magnetic unit cell is equal to, or a multiple of, the nuclear cell. The least-squares refinement procedure allows, with a simple code, the introduction of linear or quadratic constraints between the parameters.

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A description is given of Phaser-2.1: software for phasing macromolecular crystal structures by molecular replacement and single-wavelength anomalous dispersion phasing.

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An automatic procedure for recovering a complete crystal structure after a direct phasing process is described. The procedure consists mainly of a Fourier recycling method that can be implemented in any direct-methods package. The residual R value attained at the end of the process provides an estimate of the degree of success of the structure determination. The procedure can also be applied using a small molecular fragment as prior information. The procedure has been implemented into SIR92, the successor to SIR88.

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The programs WinGX and ORTEP for Windows have been distributed over the internet for about 15 years, and this article describes some of the more modern features of the programs.

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A continuous nitrogen-flow cooling device, generally applicable to X-ray diffraction studies, is described. The device works in the range 77.4 to 323.0 K with a precision of ±0.1 K and a liquid-nitrogen constant consumption rate of 0.5 l h-1 over the whole temperature range. The supply vessel is unpressurized, so that refilling can be done without any observable influence on the cooling of the sample.
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