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Atoms on Special Positions

When an atom falls on a special position -- a two-fold or higher symmetry axis -- a particular problem must be handled. Because all atoms are expanded by the symmetry of the space group when structure factors are calculated the electron density for an atom in a special position will appear too strong. For a three-fold axis the density will be three times stronger than that of a single atom in that position. To correct for this duplication the atom's occupancy should be set, and fixed, at 1/n where n is the ``fold'' of the axis. The occupancy of an atom sitting on a three-fold should be set to 1/3.


Dale Edwin Tronrud

Wed Jul 5 13:21:03 PDT 2000