Modulated and composite structures dictionary (msCIF) version 1.0.1

Category ATOM_SITE_DISPLACE_SPECIAL_FUNC

Name:
'_atom_site_displace_special_func_[]'

Definition:

   Data items in the ATOM_SITE_DISPLACE_SPECIAL_FUNC category record
   details about the displacive modulation of an atom site 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 Gao, Coppens, Cox & Moodenbaugh [Acta Cryst. (1993), A49, 141-148].

 
# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
#   In this example the displacive modulation of the O(4)
#   atom was modelled using a sawtooth-shaped function.
# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     loop_
         _atom_site_displace_special_func_atom_site_label
         _atom_site_displace_special_func_sawtooth_ax
         _atom_site_displace_special_func_sawtooth_ay
         _atom_site_displace_special_func_sawtooth_az
         _atom_site_displace_special_func_sawtooth_c
         _atom_site_displace_special_func_sawtooth_w
           O(4)  -0.270(6)  0.022(9)  0.014(2)  0.42(2)  1.07(2)

Type: null

Category: category_overview