Completion and refinement of the structure

Previous Next

As mentioned before, for small/medium-sized molecules, a preliminary diagonal least squares refinement is automatically performed at the end of the DSR process to recognize the correct solution. The least squares procedure is also suitable for the complete crystal structure refinement. Its specific features are:

  1. The full matrix or diagonal matrix may be used.
  2. 18 weighting schemes are available. If the weighting scheme contains adjustable parameters, the program refines the values to obtain a good distribution of <wΔ2> against |Fo| and resolution, and the value of the goodness of fit close to one (Spagna & Camalli, 1999).
  3. The program generates constraints for the parameters of atoms on special positions in all space groups.
  4. Automatic or guided generation of hydrogen atoms. Their contributions are included in the refinement, by allowing the positional parameters to ride on the corresponding parent atom.
  5. Floating origin is restrained automatically by setting the restrain on the sum of the appropriate coordinates.
  6. The possibility to impose conditions (constraints) or additional information (restraints).

The constrained atoms are regularized to an ideal model structure of known geometry (i.e. benzene ring) and this rigid body is refined as compact unit assuming three translational parameters and three angles which define its orientation. The method used to compute the coordinates of the model follows the approach described by Arnott & Wonacott (1966). In order to build the internal Cartesian coordinates, the program uses the ASCII file Sir2014.gru which contains models described by the Z-matrix formalism.

The following restraints are available: bond distances, bond angles, planarity.

Fourier, least squares, hydrogens and restraints tools are accessible through the Graphical User Interface (GUI).

Created with the Personal Edition of HelpNDoc: Write EPub books for the iPad