Computation of Bhat's OMIT maps with different coefficients

The OMIT electron-density-map calculation of Bhat [Bhat & Cohen (1984). J. Appl. Cryst. 17, 244-248; Bhat (1988). J. Appl. Cryst. 21, 279-281] is very effective in discovering errors in a macromolecular structure determination. A Fortran program called OMIT has been written to calculate such maps and an investigation has been carried out into which coefficients for the map calculation produce the best OMIT maps. Testing of the program on Savinase™ showed that the best overall results were obtained when |F_o| without figure of merit was used. In regions where the map is incorrect, the most interesting OMIT maps are produced when only the figure of merit, or modified SIGMAA coefficients, are used as the initial map amplitude coefficients. Thus, these tests suggest that such OMIT maps are particularly useful to reconstruct the macromolecular model in the grossly incorrect regions of the model.