Precession Pattern
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Dialog Box - called from: Display Menu

To calculate diffraction, the type numbers for atoms (Atom Data dialog), or those numbers modulo 100, must be valid atomic numbers, or must anyway correlate with the table of scattering factors in the SCATFAC.DAT file if you modify that file.

The Precession angle (mu) and Crystal-Film distance should match those of the real film to which comparison is to be made. The Precession angle determines the extent of the pattern in reciprocal space. If the Fixed Scale option (rather than Fit in Window) is selected, the pattern will be in true scale. This can be altered with the Factor, which is 1.0 by default.

The part of the reciprocal lattice shown is determined by the Horizontal axis, the Other axis and the Level. For example, for a 0kl pattern with b* horizontal, the Horizontal axis is b*, the Other axis is c*, and the Level is 0.

Reflections are shown as squares whose area is proportional to the calculated relative intensity. The intensities are normalized based on the number of electrons in the unit cell (intensity of reflection 000) and the smallest reciprocal lattice spacing in this net. The Intensity factor scales the edge length, not area, of the squares (thus it really applies to the structure factor).

To reproduce actual X-ray precession photographs, the Do Lorentz-polarization box should be checked. Omitting the Lp corrections shows the structure factors (squared), and it also allows display of the inner region in the upper levels, which is not recorded in an actual X-ray pattern. The Lp factor becomes very large near the inner and outer edges of the recorded pattern - if the value of this correction is greater than 10.0, an open symbol is used rather than a filled one. Such intensities are not reliable.

If an atom has no temperature factor, the B(iso) value in the Default temperature factor edit box will be used. Extinction and absorption are not considered.

Scattering factors used by default are those for neutral atoms from Volume 4 of the International Tables (1974 edition). Atom type numbers must be atomic numbers, or must correlate with the order of the entries in the SCATFAC.DAT file. This file may be edited to supply different scattering factors.

If the Show grid box is checked, the reciprocal lattice will be shown as a grid in red on the screen and color output, or as dotted lines in black-and-white output. Clicking with the mouse on a reflection or grid point will bring up a message box with the indices and other information.

The results of this option are shown in a graphics windows (see Types of Windows), and when this window is active, the file menu allows graphics Printing, Raster Files and Metafiles or PICT files.

For this option to work properly the space-group symbol entered either in the Space Group from Table or Custom Symmetry option must be valid for a standard setting. If it is not, it should be possible to transform to a valid setting using the program CRYSCON (Shape Software).

NOTE: Many of the sample files provided with ATOMS, especially the Basic Inorganic structures, are idealized structure types. They may have a unit cell scaled to a bond length of 1.0, or use arbitrary numbers for types instead of atomic numbers as required for diffraction. Therefore they will not always give correct diffraction results.