XFPA 98: a program for automatic structure determination and automatic refinement

The crystallographic problem: The distant goal of automatic structure determination is the development of a reliable computer program that is able to solve, re®ne and interpret crystal structures in a single run. The project of automatic structure determination was born in 1986 when, under the name XFPS, a program for automated Fourier, Patterson and superposition methods was published (Pavelcik, 1986). The program was later supplemented by graphics routines and released as a program for automatic structure determination (Pavelcik et al., 1992) of heavy-atom structures. Recently, the program has been divided into a specialized program for Patterson interpretation, including the macromolecular Patterson [difference, anomalous, etc. (Pavelcik & Dodson, 1998)], and a user-oriented `black-box' program for automatic structure determination and re®nement, presented here. tering intensity can optionally be calculated as an average of intensities from small areas of the model crystal chosen at random. This results in smooth diffraction patterns (Butler & Welberry, 1992). DISCUS can also simulate powder diffraction patterns. Two new modules were included to create disordered structures: a stacking-fault generator and a Monte Carlo simulation segment allowing the introduction of occupational as well as displacive shortrange order. Additional modules allow one to perform general symmetry operations and to transform between different crystallographic systems. The calculation and RMC-style re®nement of the atomic pair distribution function (PDF) of a given structure is another new feature of the program. Finally, it should be noted that DISCUS now supports the use of rigid molecules. This can only be a very brief listing of features; for details visit the DISCUS World Wide Web (WWW) home page (see below).