Restraints dictionary version 1.0

Category RESTR_PARAMETER

Name:
'_restr_parameter_[]'

Definition:

   This restraint, which will normally be used to restrain
   the total occupancy of an atom site, can be used to
   restrain the value of
   SUM(over the specified atoms){PARAMETER*COEFFICIENT}
   where PARAMETER will usually be the value of the
   occupancy (but other allowed quantities such as x, y
   and z can be specified in the enumeration) and
   COEFFICIENT is a user-defined number with a default
   value of 1.0. The restraint requires two loops; the
   first defines the atoms in each sum (class) and the
   second describes the properties of the class.

Examples:

These examples should be read in conjunction with the examples given for the category restr_parameter_class. Example 1. In class 1 the sites O1, O1a and O1b refer to separate sites over which oxygen is disordered, but which in total contain 0.8 oxygen atoms (as defined by _restr_parameter_class_target).

 
    loop_
    _restr_parameter_id
    _restr_parameter_class_id
    _restr_parameter_atom_site_label
    _restr_parameter_atom_coefficient
      1 1 O1   1
      2 1 O1a  1
      3 1 O1b  1

Example 2. Class 2 consists of a mixture of Na, K, Ca and Al atoms on the same site with the total occupancy set to 1.0 as defined in _restr_parameter_class_target.

 
    loop_
    _restr_parameter_id
    _restr_parameter_class_id
    _restr_parameter_atom_site_label
    _restr_parameter_atom_coefficient
      a 2 Na1  1
      b 2 K1   1
      c 2 Ca1  1
      d 2 Al1  1

Example 3. Class 3 consists of the same atoms as class 2, but by using coefficients equal to the ionic charge, the total charge on the site is restrained to 2.0. Note that the parameter restrained is still the occupancy but the use of coefficients transforms the restraint from occupancy to formal charge.

 
    loop_
    _restr_parameter_id
    _restr_parameter_class_id
    _restr_parameter_atom_site_label
    _restr_parameter_atom_coefficient
      1 3 Na1  1
      2 3 K1   1
      3 3 Ca1  2
      4 3 Al1  3

Example 4. In class 4 the y coordinate of O2 is restrained to be close to a pseudo-mirror plane at y = 0.5 (see _restr_parameter_class_target). This example may not have much practical use, but is included to show what can be done with this definition.

 
    loop_
    _restr_parameter_id
    _restr_parameter_class_id
    _restr_parameter_atom_site_label
    _restr_parameter_atom_coefficient
      1 4 O2   1

Example 5. In class 5 the positions of O3 and O4 are correlated in such a way that these atoms are displaced equal distances from the plane x = 0 [i.e., x(O3) + x(O4) = 0].

 
    loop_
    _restr_parameter_id
    _restr_parameter_class_id
    _restr_parameter_atom_site_label
    _restr_parameter_atom_coefficient
      1 5 O3   1
      2 5 O4   1

Type: null

Category: category_overview

		contents
		indexes
		sample pages
		purchase
		CIF dictionaries

Crystallographic Information Framework

Restraints dictionary version 1.0

Category RESTR_PARAMETER

International Tables for Crystallography

Volume G: Definition and exchange of crystallographic data

Edited by S. R. Hall and B. McMahon

IUCrData