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Real Space Asymmetric Units

This table lists the real space asymmetric units for all the space groups handled by TNT. The unique portion of the data for a space group is determined by the lattice class, the point group, and sometimes, the orientation.

Because the FFT algorithm cannot handle all types of symmetry the Fourier synthesis calculations for some space groups are actually performed in a lower symmetry group. This requires that the real space asymmetric unit be larger by a factor of the multiplicity of the symmetry being ignored. For example, direct three-fold axes can never be used to speed the calculation of an FFT. Therefore space group P23 is handled as though it were space group P222. The amount of map required is a factor of three larger that that expected for this group.

The ``Additional Restrictions'' column lists the additional factors that the sampling rate must contain. No sampling rate, in any direction, may have a prime factor larger than 17. The smaller the largest prime factor is the more efficient the FFT calculations will be.





Dale Edwin Tronrud
Thu Jul 6 23:24:57 PDT 2000