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The category Computer Program Abstracts provides a rapid means of communicating up-to-date information concerning both new programs or systems and significant updates to existing ones. Following normal submission, a Computer Program Abstract will be reviewed by one or two members of the IUCr Commission on Crystallographic Computing. Itshould not exceed 500 words in length and should use the standard format given on page 189 of the June 1985

A low-temperature high-resolution X-ray and neutron diffraction study of charge distribution in the superacid dipicrylamine (DPA) has been carried out. This structure exhibits interesting weak interactions including bifurcated N-H‚‚‚O and C-H‚‚‚O H-bonds which consolidate it. The corresponding interaction lines appear to be modified in the vicinity of the critical points of the covalent bonds of the hydrogen atoms. The presence of six nitro groups generates some short N‚‚‚O contacts. On the basis of the experimental interaction paths, an attractive interaction is found, in agreement with the prediction of such an effect from earlier model ab initio calculations performed for simple dimers. For these weak interactions, a linear relationship is found between the charge density at the interaction critical points and the N-O distance. † Dedicated to Prof. T. M. Krygowski on his 65th birthday.

The great majority of the known crystal structures built up only of centred trigonal prisms can be classified according to (a) the number of different prism-axis directions, (b) the dimensionality of the A1B2-type fragments, (c) the value of the prism-linkage coefficient, (d) the ratio of the number of centred prisms to the number of W-type segments and (e) the numbers of prism-centre sites with different waist-contact type. Many ternary rare-earth-transition-metal silicide, germanide and gallide structures are characterized by transition-metal and Si-, Ge-or Ga-centred trigonal rare-earth prisms. Atom ordering on the prism-centre sites is to be expected whenever the transition element and the main-group element tend to induce different rare-earth prism shapes. The atom ordering is characterized by the absence of transition-metal-rare-earth and transition-metal-transition-metal waist contacts. This ordering principle not only explains the structures and compositions of the known ordered compounds, but also allows a prediction of the compositions for full order of compounds where a random distribution of prism sites has been reported previously. The application of this ordering principle is demonstrated on earlier-reported Ce14NisSi9, for which at the predicted, slightly different, composition CelaNi6Sill a fully ordered structure is verified.