In many structures, groups of atoms (molecules or coordination polyhedra) have a well established structure, are not completely independent, and therefore form a relatively rigid unit.  Typical examples are the cyclopentadienyl anion or the benzene ring.  Rigid bodies have been a common tool in single crystal X-ray diffraction for more than 30 years, and are especially valuable when the quality of the data is low, the ratio of observations to parameters is low,  and/or the structure is very complicated (e.g. proteins).  According to Scheringer there are several general advantages for using rigid bodies:

• Since the group is forced to shift as a complete unit, meaningless changes cannot occur.
• The number of refinable parameters can be drastically reduced and they can be determined with much higher accuracy.  This is in particular useful in the case of powder data, since the ratio of the number of independent observations (Bragg intensities) to refinable parameters is typically very low.
• The range of convergence to the correct structure is much larger than in the normal refinement.
• Hydrogen atoms can be included in the refinement process at an early stage.  Only their relative positions in respect to the other atoms are needed.  With powder data they can only be found directly in very special cases although their contribution to the profile is clearly measurable.
• The thermal parameters refer to the group as a whole.  The use of TLS matrices allows anisotropic refinement of the translational and librational parts of the temperature factor.

Furthermore, it is often impossible to refine individual atomic positions if disorder occurs.  By using rigid bodies it might be possible to model disorder even in the case of powder data.  The advantages which led to the introduction of rigid bodies in single crystal analysis are even more valid for powder data.  Interestingly, very little has been done so far in this field and only a few Rietveld refinement programs offer a rigid body option.

The aim of this paper is to give a practical guide to the powder community to explore the possibilities of rigid bodies in Rietveld refinement.  A pre-version of this paper was originally written for a manual of a workshop on ”Ab initio structure determination of molecular solids from powder diffraction” held at Bayreuth, September 25.-28. 1997.  Although there are many different ways to set up a rigid body, the basics are similar.  In the following paper, the notation of the widely used Rietveld refinement program GSAS is used.