Modulated and composite structures dictionary (msCIF) version 1.0.1

Category ATOM_SITE_OCC_SPECIAL_FUNC

Name:
'_atom_site_occ_special_func_[]'

Definition:

   Data items in the ATOM_SITE_DISPLACE_OCC_SPECIAL_FUNC category
   record details about the occupational modulation of a given atom
   or rigid group in a modulated structure when it is not described
   by Fourier series. Special functions are effective in some cases
   where the modulations are highly anharmonic, since the number of
   parameters is drastically reduced. However, they are in general
   discontinuous or with discontinuous derivatives and therefore
   these functions describe an ideal situation that never occurs in
   a real modulated crystal. Up to now, only a few types of special
   functions have been used and all of them come from the JANA suite
   of programs.  Although this approach is far from being general,
   it has the advantage that the functions are tightly defined and
   therefore the atomic displacements and occupations can be
   calculated easily. In this dictionary, only the special functions
   available in JANA2000 have been included.
   These are:

     (1) Sawtooth functions for atomic displacive modulation along
        x, y and z.
     (2) Crenel functions for the occupational modulation of atoms
        and rigid groups. Both of these only apply to
        one-dimensional modulated structures.

Example:

Example 1 - extracted from Elding-Pont\'en, M., Stenberg, L., Lidin, S., Madariaga, G. & P\'erez-Mato, J.M. [Acta Cryst. (1997), B53, 364-372].

 
# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
#   In this example the occupational modulation of the Mn(2)
#   atom was modelled using a square-wave crenel function.
# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     loop_
         _atom_site_occ_special_func_atom_site_label
         _atom_site_occ_special_func_crenel_c
         _atom_site_occ_special_func_crenel_w
             Mn(2)    0.25    0.623(4)

Type: null

Category: category_overview