PHASE COMBINATION AND CROSS-VALIDATION IN ITERATED DENSITY-MODIFICATION CALCULATIONS

A variety of density-modification techniques are now available for imp roving electron-density maps in accordance with known chemical informa tion. This modification must, however, always be constrained by consis tency with the experimental data. This is conventionally achieved by a lternating cycles of map modification in real space with recombination with the experimental data in reciprocal space. The phase recombinati on is based upon the assumption that the density-modified map may be t reated as a partial model of the structure which contains information independent of the experimentally derived phases. This assumption is s hown to be incorrect, and an alternative procedure is investigated whi ch as a side effect allows calculation of a free R factor.