The frequency check, utilizing three-dimensional super positions of the Patterson and vector verification maps, assigns relative values to positions in electron-density space which are consistent with both the Patterson function and the space group symmetry of the crystal system. These relative values--which represent the extent of peak coincidence between a Patterson and electron density function— facilitate the choice of the first tentative atom position, a decisive factor in the success of the vector verification procedure. Employing a bivalued Patterson map, each three-dimensional superposition result¬ing in a relative value requires one to five seconds on a large solid state computer.
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