Angela Altomare

SIR97: a new tool for crystal structure determination and refinement

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

Completion and refinement of crystal structures with SIR92

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.

EXPO: a program for full powder pattern decomposition and crystal structure solution

EXPO is the integration of two programs, EXTRA and SIRPOW.92, the first devoted to full powder pattern decomposition and the second to the solution and refinement 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 refinement.

E-map improvement in direct procedures

The quality of an E map is usually affected by systematic and/or random phase errors, by amplitude truncation effects in the series representation of the electron density, by the experimental uncertainty in the estimation of |E| and by the intrinsic nature of the Fourier coefficients used (i.e. the Es). It is shown that simple supplemental calculations can improve the quality of an E map. Large molecular fragments can often be localized in the new map even when the original one is not easily interpretable.

EXPO2009: structure solution by powder data in direct and reciprocal space

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.

EXPO2013: a kit of tools for phasing crystal structures from powder data

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.

EXTRA : a program for extracting structure-factor amplitudes from powder diffraction data

Extraction of structure-factor amplitudes from a powder diffraction pattern is not a straightforward procedure. Peak overlapping and background estimation are the main obstacles to the process : they may introduce strong correlations among reflection intensities and heavy errors in their estimates. The program EXTRA is described, which, on the basis of the Le Bail algorithm, is able reliably to estimate the structure-factor amplitudes in a fully automatic way.

Early finding of preferred orientation: a new method

Pattern 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.

Automatic structure determination from powder data with EXPO2004

EXPO2004 is the updated version of the EXPO program [Altomare et al. (1999). J. Appl. Cryst. 32, 339–340]. The traditional steps of the ab initio powder solution process are performed automatically: indexing, space-group determination, decomposition of the pattern for extracting the observed structure-factor moduli, structure solution by direct methods, model refinement by Rietveld technique. Special strategies may be applied to improve both the estimates of the extracted structure-factor moduli and the quality of the structure model. In addition, the use of special procedures exploiting available supplementary information on molecular geometry can be successfully adopted. The graphical interface has also been improved.

New techniques for indexing: N-TREOR in EXPO

Indexing of a powder diffraction pattern is still a critical point in procedures aiming at solving crystal structures from powder data. New code has been associated to the program TREOR90 in order to define an efficient peak search procedure, to modify the crystallographic decisions coded into TREOR90 to make it more exhaustive, to refine the selected unit cell automatically, and to make the entire procedure user friendly, via a graphical interface. The new program, called N-TREOR, has been integrated into the package EXPO to create a suite of programs able to provide a structural model from the analysis of the experimental pattern. N-TREOR is also available as a stand-alone program.