diffraction

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Purpose

The diffraction command is used to calculate the X-ray, neutron, or electron scattering pattern for a molecular system. The system may comprise one or several isolated molecules or may be of the periodic type, as used to model bulk liquids and amorphous solids. The structure used may be either the current molecular system or a series of structures obtained from an .arc or .his file. Output is directed to a graph file in the standard .tbl format.

Syntax

diffraction ?keyword = value?...


Keyword          Values   Default               Meaning


rad*iation_type  xray     xray         Perform calculation for X-ray radiation.
                 electron              Perform calculation for electron beam.
                 neutron               Perform calculation for neutron beam.

lim*it_type      q        q            Use q = 4  sin / for calculation (Å^-1).
                 s                     Use s = 2 sin / (Å^-1).  
                 t*heta                Use scattering angle = 2  (degrees).

wave*length      real     0            Radiation wavelength--required only for 
                                       limit_type = theta.

limit_min*imum   real     0            Minimum limit for q, s, or scattering 
                                       angle, as appropriate.

limit_max*imum   real     0            Maximum limit for q, s, or scattering 
                                       angle, as appropriate.

frame*_definition
                 string    --          Name of file containing a specification 
                                       of a Discover.arc or .his file and 
                                       frames to be used.

sub*set          string   --           Name of the atom set to be used in the 
                                       analysis. The atom set must be present 
                                       in the .mdf file; if not, the analysis 
                                       is performed on the entire molecular 
                                       system.

table            Boolean  false        Write output to a table file.

file*name        string  run_name.tbl  Name of .tbl file to receive output.

Description

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The diffraction command is used to perform analysis on the current molecular system or on structures that were previously saved in .arc or .his files.

In general, it is used to perform analysis on all the atoms composing the system, and this is indicated by omitting the subset keyword and name. However, if desired, a subset of the atoms can be indicated. This is useful for reducing the computation time when the contributions of certain atoms are negligible (e.g., H atoms for X-ray scattering).

Intensities are given as I / I₀, where I₀ denotes the intensity at zero angle. Limits may be given in units of s = 2 sin / , q = 4 sin / , or as the scattering angle 2 (in which case specification of the wavelength, in angstroms, is mandatory).

The following restriction applies to the use of this command:

For periodic systems, the cell must be orthorhombic.

All output is written to standard .tbl files, in the form I / I₀ vs. S, q, or 2

, as appropriate.

For details on theory and application relevant to this command, please consult the generic Diffraction pulldown documentation. This can be found in the Polymer User Guide and the Catalysis User Guide.

Example 1

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diffraction frame_definition = argon.fdef \ 
 	 radiation_type = xray limit_type = q \ 
 	 limit_minimum = 0.1 limit_maximum = 5.0 \ 
 	 +table filename = argon_xray.tbl

This example of the diffraction command calculates the scattering pattern for a modeled argon system, using configurations described in the file argon.fdef. The actual configurations are stored in a .his or .arc file, as indicated in the frame definition file.

Example 2

diffraction radiation_type = electron \ 
 	 wavelength = 0.1 limit_type = theta \ 
 	 limit_minimum = 10 limit_maximum = 60 \ 
 	 +table filename = ccl4_electron.tbl

This example of the diffraction command calculates the electron diffraction curve for the current system, a carbon tetrachloride molecule. The results are given as a function of the scattering angle.

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