Restraints on displacement parameters

Two of the three restraints on displacement parameters (DELU, SIMU) take into account the fact that atoms, which are bound to one another, move similarly, both in direction and amount (Hirschfeld 1976 Didisheim and Schwarzenbach 1987). The third one (ISOR) encourages approximate isotropic behaviour for otherwise anisotropically refined atoms. Both SiMU and DELU are based on physically very sensible assumptions and can be used on all or almost all atoms in a model when the data to parameter ratio is low or other problems with the refinement make this seem desirable. SIMU, however, should not be applied to very small ions, isolated atoms and atoms that are part of freely rotating groups. 

This rigid bond restraint is applied to all bonds connecting atoms on the same DELU instruction. It restrains the anisotropic displacement parameters of two atoms in the direction of the bond between them to be equal within a given standard uncertainty si (default value 0.01 A ). When appropriate, the same restraint is applied to 1,3-distances, employing the standard uncertainty s2 (default value is also 0.01 A ). The syntax is 

If no atom names are given, all non-hydrogen atoms are assumed if si but not s2 is specified, s2 is assumed to possess the same value as si. The use of DELU is explained and illustrated in detail in Chapter 5. 

It can be assumed that atoms that are bound to one another would move in similar directions with approximately similar amplitudes. With the syntax 

ISOR restrains the components of anisotropically refined atoms to behave approximately isotropically within a standard uncertainty of s, or st for terminal atoms (default values 0.1 and 0.2 A ). The syntax is as follows  

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