This sublevel of DISCUS calculates the Fourier transform of the given crystal structure. The algorithm to speed up the explicite Fourier is based on the program 'diffuse' by B.D. Butler. See also: B.D. Bultler & T.R. Welberry, (1992). J. Appl. Cryst. 25, 391-399.
The program allows to calculate X-ray and neutron scattering including isotropic temperature factors and anomalous scattering. The program also allows to subtract the structure factor of the average structure from the calculated diffraction pattern.
Further help topics are:
Valid commands at this level are:
@ ! execute a macro file (see main help level) = ! assigns the value to a variable (see main help level) abs ! defines the coordinate of the abszissa delf ! defines the anomalous scattering corrections for an element disp ! switches anomal dispersion on/off echo ! echo a string (see main help level) eval ! Evaluates an expression for interactive check exit ! terminates Fourier level help ! help to fourier commands (see main help level) layer ! defines the layer in reciprocal space to be calculated ll ! defines the lower left corner in reciprocal space lots ! defines the crystal volume to be used for the Fourier transform lr ! defines the lower right corner in reciprocal space na ! defines the number of points along the abszissa neut ! switches to neutron diffraction no ! defines the number of points along the ordinate ord ! defines the coordinate of the ordinate run ! starts the calculation scat ! defines the scattering factor for an element set ! sets Fourier mode show ! shows the current settings for the Fourier transform system ! Executes operating system command (see main help) temp ! defines whether temperature coefficients are to be used ul ! defines the upper left corner in reciprocal space wait ! Waits for user input (see main help level) wvle ! Defines the wave length to be used xray ! switches to X-ray calculation
Defines which the component of the vector parallel to the abszissa is written to the output file.
Defines for the element <name> or the scattering curve number <number> the real <f'> and the imaginary <f''> correction terms for the scattering factors.
switches anomal dispersion on /off. If <switch> is "anom" anomalous dispersion will be calculated, if the switch is omitted, no anomalous dispersion is calculated.
sets the lower left, lower right and upper left corner of the Fourier plane to be calculated and sets the number of data points along the horizontal and the vertical
Sets the lower left corner of the Fourier plane to be calculated.
The program allows to Fourier transform large crystals by averaging the scattering from smaller volumes selected at random. This allows to avoid very long correlations which might occur if the complete crystal scatters in phase.
If this option is not wanted and the complete crystal should be transformed at once, this option can be turned off by the command 'lots off'. Otherwise the following parameters must be specified: First the lot shape. The choices are "box" or "ellipsoid". Usually the first letter of the parameter is sufficient. The following parameters <lx>,<ly> and <lz> describe the size of the subvolumes in unit cells. The parameter <n> specifies the number of such volumes to be averaged. The last parameter can be "yes" or "no" and selects if periodic boundary conditions should be applied.
The lot size should be slightly larger than the longest correlations present in the model crystal studied. The number of lots to be averaged should be set to a value that complete crystal will be covered once on average. WARNING: If the command 'purge' is performed or a structure is read where the atoms are not listed in the DISCUS order this command should be set to 'lots off'.
Sets the lower right corner of the Fourier plane to be calculated.
Sets the number of data points that will be calculated along the abszissa.
Switches to neutron diffraction
Sets the number of data points that will be calculated along the ordinate.
Defines which the component of the vector parallel to the ordinate is written to the output file.
starts the Fourier calculation. The user is given a rough estimate of the time it will take.
Defines for the element <name> or the scattering curve number <number> a new scattering factor in the exponential form.
The Fourier level of DISCUS allows to subtract the average structure factor <F> by setting the parameter <per> to a value not equals zero. The value of <per> sets the percentage of the crystal to be used to calculate <F>.
The average is calculated by putting <per> % of the crystals atoms into a single unit cell which is Fourier transformed and multiplied with the interference function of the lot shape. WARNING: If the command 'purge' is performed or a structure is read where the atoms are not listed in the DISCUS order the average structure will not be calculated correctly !
Shows the current settings for the Fourier transform file
sets whether temperature coefficients are to be used or not. Valid values for <switch> are "use" and "ignore". The first two letters of the switch are sufficient.
Sets the upper left corner of the Fourier plane to be calculated.
Defines the wave length to be used. Anomalous dispersion will only be calculated if the wave length is given as symbol.
Switches to X-ray calculation