The model of the

A method for empirical determination of TDS (thermal diffuse scattering) corrections in X-ray structure analysis is developed. Thermal diffuse scattering is introduced into the model suggested earlier [Chulichkov et al. (1987). Soy. Phys. Crystallogr. 32, 649-653; Laktionov, Chulichkov, Chulichkova, Fetisov, Pyt'ev & Aslanov (1989). J. Appl. Cryst. 22, 315-320] for measuring the Bragg reflection intensity profile with a four-circle diffractometer. The improved model in combination with the mathematical reduction method enables the extraction of TDS and background intensity values from the experimental intensity profile. Thus the problem of TDS correction in crystal structure refinement is solved without knowledge of the sample's elasticity constants. The comparison of TDS corrections obtained by this method with those calculated from elasticity constants for hexamethylenetetramine, C6H2N4, and sulfur, $8, crystals shows good agreement.

The software package PPXA (programs for precise X‐ray analysis) is described, which provides crystallographers with increased precision and reliability of the results of X‐ray analysis. The package includes programs for: testing the quality of an automatic four‐circle X‐ray diffractometer before X‐ray experiments; optimization of X‐ray diffraction experiments; preliminary processing of experimental data; precise structure analysis up to electron density distribution. The programs for diffractometer testing allow the experimenter to check the stability and precision of the instrument, the homogeneity of the X‐ray beam and the detector sensitivity. The programs for preliminary data processing include: corrections for absorption, taking into account the intensity distribution in the X‐ray beam cross section; correction or elimination of the simultaneous reflection effect; background subtraction, correcting the effects of the scan‐interval cut‐off; correction for thermal diffuse scattering (TDS) for crystals in cases of both known and unknown elasticity constants. The programs for precise crystal structure refinement include options for the determination of primary‐ and secondary‐extinction parameters, atomic coordinates, thermal‐vibration parameters in harmonic and anharmonic approximations, occupation of atomic positions, atom‐core and valence‐shell occupation parameters. The programs were written for a MicroVAX‐II computer with a VAX/VMS operating system and designed to work with CAD‐4 diffractometers and the SDP‐Plus software package. A suitable dialogue with a menu system and detailed user's instructions are available.

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