Molecular Replacement Real Space Averaging

Structure determination of macromolecules often depends on phase improvement and phase extension by use of real space averaging of electron density related by noncrystallographic symmetry. Although techniques for such procedures have been described previously (G. Bricogne, Acta Crystallogr. A32, 832-847, 1976; J. E. Johnson, Acta Crystallogr. B34, 576577, 1978), modern computer architecture and experience with these methods have suggested changes and improvements. Two unit cells are considered: (1) the p-cell corresponding to the actual crystal structure(s) being determined (there would be more than one of these if the molecule crystallizes in more than one crystal form) and (2) the h-cell corresponding to the molecule in a standard orientation with respect to which the molecular symmetry axes are defined. Averaging can proceed entirely within the p-cell, referring to the h-cell only in as far as knowledge of the molecular symmetry is required. It is also possible to place the averaged..