DISCUS is intended as a versatile tool to simulate crystal structures and the corresponding intensity distribution in reciprocal space. The program offers several features that enable the user to easily generate a structure and to introduce various defects. The main intend is to simulate defect structures, the program, however, is not limited in that respect. Ideal structures can be simulated as well and usually will form the basis from which the defect structure is developed. ============================================================================== The main features of the program include: -- Reading an arbitrary structure Reading an asymmetric unit expanding to a full crystal by use of the space Group symbol Free generation of a structure The program can read a whole crystal or the asymmetric unit of a unit cell. The latter is expanded to the whole unit cell by use of the space group symbol. The free generation of a structure sets a framework in which individual atoms can be placed to form any (aperiodic/periodic) structure -- Tools to modify the structure Several tools are available to modify the structure. Individual atoms can be added, moved, deleted, modified The whole crystal can be modified by introducing thermal displacement wave functions displacement and density waves with several wave functions. microdomains any type of defect distributed throughout the host structure. stacking faults creating stacking faults -- Tools to analyse a structure Calculating the average structure including mean deviations from the average position. Computing of bond length distributions and neighbouring frequencies. -- Fourier Transform The Fourier transform segment of the program allows the user to calculate the intensity distribution along a line or on any plane through reciprocal space. The resolution in reciprocal space can be chosen by the user. The Fourier transform is calculated from explicit summation of the standard kinematical formula. Both, the X-ray and neutron the form factors (scattering lengths) are tabluated in the source code. Anomalous scattering can be taken into account. New in version 3.0 is the possibility to subtract the average structure factor . Also new in 3.0 is the possibility to calculate the intensity as average of the intensity of small randomly selected volumes of the model crystal. This created smoother diffraction patterns free of high frequency oszillations (-> coherence). -- Inverse Fourier / Patterson This segment reads one or several layers of reciprocal space and calculates the Patterson or inverse Fourier transform. -- Reverse Monte Carlo Complete Reverse Monte Carlo simulation sublevel which allows to modify a model structure according to experimental diffuse scattering data. -- Symmetry calculations A new sublevel in version 3.0 of DISCUS allows symmetry calculations and transformations in the given space group. -- Output generated The result of the Fourier transform may be written in ASCII in several output formats, to be displayed by standard visualization programs or to be printed directly. The structure generated may be saved for later use. The structure or parts thereof may be written in a format suitable for displaying the structure. A special format used with KUPLOT allows easier structure plots as in previous versions. -- Command language The program uses a command language to interact with the user. No predefined requirement exists for a given sequence of commands. The commands can be typed at the DISCUS prompt, or read from a macro file. Macro files may be written by any editor or learned during execution. -- Build in FORTRAN interpreter The command language includes a FORTRAN style interpreter that allows the user to program loops and logical structures. Several real and integer variables as well as structural variables can be used to design the intended defect structure. You can use the interpreter to modify individual atoms and thus generate any desired defect structure. -- File names A special feature of the FORTRAN interpreter is an automatic file name generation process. This allows you to perform multiple simulations that can all be saved under individual names. -- On-line help On line help is provided which gives the user information on any of the DISCUS commands as well as examples for typical DISCUS sessions. -- Error Messages The program generates error messages for (hopefully) all erroneous situations that should provide enough information to the user in order to correct the errors that have occurred. -- Manual & tutorial The distribution contains a manual, a HTML verison of the online help and an interactive tutorial to help you getting started with DISCUS. =============================================================================== The program is written in standard Fortran-77 and can easily be adapted to any specific Fortran compiler. The program has been installed on several different platforms: SUN-workstations HP -workstations SGI-workstations LINUX VMS (VAX and ALPHA) MS-DOS =============================================================================== For further information or suggestions please contact Dr. Reinhard Neder Institut fuer Kristallographie, LMU Phone: (+49 89)2394 4314 Theresienstr. 41 Fax: (+49 89)2394 4334 D-80333 Muenchen, Germany Email: neder@yoda.kri.physik.uni-muenchen.de or Dr. Thomas Proffen Research School of Chemistry Phone: (+61 6) 249 3579 Australian National University Fax: (+61 6) 249 0750 Canberra, ACT 0200, Australia Email: proffen@rsc.anu.edu.au