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DISCUS                                                            Version 3.2
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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.

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New features of DISCUS 3.2 :
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- Calculation of powder diffraction patterns from given structure

- Calculation of the atomic pair distribution function (PDF) of a given
  structure and RMC style refinement to fit an observed PDF.

- Intensities can be saved in SHELXL format.

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Main features of DISCUS in general :
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-- Reading an arbitrary 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

      DISCUS supports the use of rigid molecules (new in 3.1)

-- 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 
           monte carlo simulation (new in 3.1)
               creating short range order via MC simulations

-- Tools to analyse a structure

      Calculating the average structure including mean deviations from the
      average position. Computing of bond length distributions and 
      neighbouring frequencies. Occupational as well as displacement 
      correlations and complete correlation fields can be computed.

-- 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 
      tabulated 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 <F>. 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 oscillations (-> coherence).

      As a new feature in version 3.2 one can calculate the powder 
      diffraction pattern.

-- Inverse Fourier / Patterson

      This segment reads one or several layers of reciprocal space and
      calculates the Patterson or inverse Fourier transform. 

-- Atomic Pair Distribution Function (PDF)

      DISCUS can calculated the PDF of a given structure. The program
      allows one to select a Q(max) value and various other correction
      parameters for the PDF calculation. DISCUS can calculate the total
      as well as partial and/or differential PDFs.

-- Reverse Monte Carlo

      Complete Reverse Monte Carlo simulation sublevel which allows to
      modify a model structure according to experimental diffuse 
      scattering data.

      In addition to observed scattering data, the user can also refine
      a PDF using RMC (Note the new program PDFFIT for PDF refinements !)

-- Symmetry calculations

      A new sublevel in version 3.0 of DISCUS allows symmetry calculations
      and transformations in the given space group.

-- Unit cell transformations

      A new sublevel in version 3.1 of DISCUS allows unit cell trans-
      formations.

-- 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 documentation in form of a command 
      reference and users guide as well as a online help function 
      and an interactive tutorial to help you getting started with
      DISCUS.

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