SIR97 is the integration of two programs, SIR92 and CAOS, the ®rst devoted to the solution of crystal structures by direct methods, the second to re®nement via least-squares±Fourier procedures. Several new features have been introduced in SIR97 with respect to the previous version, SIR92: greater automatization, increased ef®ciency of the direct methods section, and a powerful graphics interface. The program also provides publication tables and CIF ®les.

The category Computer Program Abstracts provides a rapid means of communicating up-to-date infor Appl. CrysL (1985). 18,[189][190].J. Appl. CrysL (1994) The crystallographic problem: Our recent developments in direct-methods theory and in crystal-structure refinement persuaded us to develop a fully automated structure-determination program that is able to solve the phase problem and to refine atomic parameters in a single computer run. Method of solution:The method for automatic structure determination is based on the representation theory (Giacovazzo, 1977(Giacovazzo, , 1980. One-phase structure seminvariants and three-phase structure invariants are estimated via their second representation, two-phase seminvariants and four-phase invariants via their first representation. SIR92 is the heir of SIR88 (Burla, Camalli, Cascarano, Giacovazzo, Polidori, Spagna & Viterbo, 1989), and it retains most of the original features (Le. treatment of pseudotranslational symmetry, use of prior information, tangent weighting schemes etc.). In SIR88, one-and two-phase seminvariants and positive estimated three-phase invariants could be actively used in the phasing process. Negative estimated triplet and quartet invariants were used as a figure of merit (FOM) together with psi-zero triplets. In SIR92, negative estimated triplet and quartet invariants and psizero triplets are actively used in the © 1994 International Union of Crystallography Printed in Great Britain -all rights reserved phasing process and proved to be much more robust and efficient. The various trial solutions can be obtained either by magic-integer permutation (Main, 1978) or by random phases (Baggio, Woolfson, Declercq & Germain, 1978). The best trial solution, selected by powerful FOM's, is automatically processed through a cyclic procedure combining structure-factor calculation, least squares and 2Fo-Fc Fourier synthesis. The final outcome is a set of refined atomic parameters (x, y, z and isotropic B values) associated with suitable atomic species. The residual R value is calculated for user usefulness (final value usually between 0.08 and 0.15). The program runs in default mode when supplied with a minimum amount of information (space-group symbol, cell parameters, unit-cell chemical content and reflections) but documentation has been produced to allow the user to change default values. If a graphic device is available, the user can follow the structure solution and refinement on the screen. In the final stage, a menudriven interface is available in order to study molecular geometry and restart the refinement.Software environment: The program is written in standard Fortran77. A module written in C is supplied for interface with X-window or DEC-Window terminals for graphics. Besides a C compiler, an Xlib library is also needed. Two ASCII files are associated with the scattering program: the first contains coefficients for calculating scattering factors, the second, necessary for graphics, contains on-line help.Hardware environment: The program runs on Unix and...

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. Symbols and abbreviationsm: number of symmetry operators of the point group. N: number of non-H atoms in the unit cell. t: number of non-H atoms in the asymmetric unit. NLAR: number of large normalized structure factors selected for active use in a direct procedure. NEXP: number of large normalized structure factors immediately following the NLAR reflections, to which phase expansion procedures can safely be applied.

EXPO2013, the heir of EXPO2009, has been enriched by a variety of new algorithms and graphical tools aiming at strengthening the individual steps of the powder structure solution pathway. Particular attention has been addressed to the procedures devoted to improving structural models provided by direct methods in ab initio approaches. In addition, a new procedure has been implemented, working in direct space, which may be chosen by the user as an alternative to the traditional simulated annealing algorithm.

EXPO is the integration of two programs, EXTRA and SIRPOW.92, the ®rst devoted to full powder pattern decomposition and the second to the solution and re®nement of crystal structures. The program is able to exploit the prior information obtained during the crystal structure solution process for improving the classical decomposition. EXPO also allows preliminary cycles of Rietveld re®nement.

The program EXPO2009 is the evolution of EXPO2004 [Altomare, Caliandro, Camalli, Cuocci, Giacovazzo, Moliterni & Rizzi (2004). J. Appl. Cryst. 37, 1025–1028]. EXPO2009 performs all the steps of ab initio structure solution by powder data: indexing, space‐group determination, estimation of the reflection integrated intensities, structure solution by direct/Patterson methods and/or by a direct‐space/hybrid approach, and model refinement by the Rietveld technique. New procedures have been introduced in EXPO2009 for enhancing the structure solution process, particularly in the case of low‐resolution data and/or organic compounds, when traditional approaches like direct methods may fail. The EXPO2009 graphical interface has been optimized and made very user friendly.

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So far, we have derived the equations in orientation analysis as well as in quantitative phase analysis based on the 2D X-ray diffraction theory. The use of 2D X-ray diffractometry is providing new prospects for the study of the microstructure of crystalline materials of both fundamental and technological importance. As these become more widespread, we are certain that these studies will be of great help in understanding X-ray diffraction analysis. ReferencesCONe;, Q. Z. (1993). Acta Cryst. A49, 23-28. CONG, Q. Z. (1994 AbstractPattern decomposition programs are able to derive, from the measured powder diffraction patterns, estimates of the square moduli of the structure factors. A method is described that, by statistical analysis of the normalized structure-factor moduli, is able to obtain information about the possible presence of preferred orientation. Cylindrical symmetry of the specimen is necessary. The method proves to be efficient and quite useful for the application of direct methods to powder data.