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This chapter discusses the rotation function calculation with the GLRF program. The calculation of rotation functions represents the first step in structure determination by the molecular-replacement method. Rotation functions can be used to determine the orientation of a noncrystallographic axis in a crystal or the orientation of a search model relative to the molecules in a crystal. In the second step, rotational parameters can be used to determine the position of the noncrystallographic symmetry in the crystal or the position of the search model in the crystal. The information on the orientation and position of a search model leads to an initial solution for the crystal structure. Finally, the orientation and position of the noncrystallographic symmetry axes can be used to determine an initial phasing set, as well as to improve and extend the phase information.