DISCUS, a program for diffuse scattering and defect structure simulations - Update

The crystallographic problem: In contrast to the determination of the average structure based on Bragg scattering, there is no general procedure to extract information about the disorder of a material from its diffuse scattering. Two years ago, we first published the program DISCUS(Proffen & Neder, 1997), which allows the creation of defect structures and the simulation of diffuse scattering. The program can also be successfully used to aid teaching of diffraction physics (Neder & Proffen, 1996). This abstract gives an overview of the changes and enhancements between the published version 2.01 and the current version 3.2. Method of solution: The program DISCUS allows one to create structures from the contents of an asymmetric unit using the space-group symbol. Structures can be modified using the Fortranstyle interpreter or built-in functions of the program. Disordered structures can be analyzed, e.g. by calculating bond-length distributions, and the Fourier transform can be calculated. DISCUS includes a module for reverse Monte Carlo (RMC) simulations. We will focus here on the new features of the program. The Fourier transformation is now calculated about six times faster and allows the subtraction of the average structure factor (F). The scattering 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 short-range order. Additional modules allow one to perform general symmetry operations and to transform between different crystallographic systems. The calculation and RMC-style refinement 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). Software environment: DISCUS is written in standard Fortran 77 with the exception of the command-line interface which is written in C. The program is distributed as source code and a Fortran and C compiler are required for the installation. However, executable files may be obtained from the authors. Hardware environment: The program has been successfully used on various UNIX platforms, including LINUX, and under VMS. Although DOS binaries may be obtained from the authors, a UNIX-type platform is recommended. The memory requirements strongly depend on the needs of individual cases, such as the model crystal size. A typical setup requires about 10-16 MByte of memory. The necessary hard-disk space for the installation is about 10 MByte. Program specification: Performance of DISCUS depends strongly on the computer used. The calculation of a 121 × 121 point array in reciprocal space from a crystal containing 32000 atoms takes about 45s on a Pentium II computer running LINUX. Documentation: The program is distributed with a detailed users guide explaining the basic concept of the program and with a command reference containing information about every command. Furthermore, DISCUS has an online help function, allowing the user to obtain information about a specific command while running the program. Availability: DISCUS is available via the WWW at http://www.pa.msu.edu/cmp/billinge-group/discus/discus.html or at http://www.uni-wuerzburg.de/ mineralogie/crystal/discus/discus.html. The program can be used free of charge.