Restraints Rietveld refinement

A thorough reader should be able to verify the correctness of this statement by attempting Rietveld refinement without imposing soft restraints. 

For the minimization function in a Rietveld refinement including stereochemical restraints, the terms are Y - powder pattern, a - bond angles. 

The crystal structures of proteins represent an extreme in the number of atom positions needed to describe them compared to those structures more commonly studied by powder diffraction. For example, the well-known tetragonal crystal structure of hen egg white lysozyme has 1001 nonhydrogen atoms within the protein molecule another 100 or so water molecules and salt ions are also present. This gives over 3000 atomic x,j,z coordinates. Nonetheless, a Rietveld refinement of these structures from powder diffraction data can be performed by extending the suite of restraints to include all stereochemical features that show characteristic values.The suite of restraints given in Equation (24) is then ... 

Nowell, H., J.P. Attfield, and J.C. Cole. 2002. The use of restraints in Rietveld refinement of molecular compounds A case study using the crystal structure determination of tryptamine free base. Acta Crystallogr. 586 835-840. 

R.B. Von Dreele, Combined Rietveld and stereochemical restraint refinement of a protein crystal structure, J. Appl. Cryst. 32, 1084 (1999). 

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