Advances in the EDM-DEDM procedure

The DEDM (difference electron-density modification) algorithm has been described in a recent paper [Caliandro et al. (2008), Acta Cryst. A64, 519-528]: it breaks down the collinearity between model structure phases and difference structure phase estimates. The new difference electron-density produced by DEDM, summed to the calculated Fourier maps, is expected to provide a representation of the full structure that is more accurate than that obtained by the observed Fourier synthesis. In the same paper, the DEDM algorithm was combined with the EDM (electron-density modification) approach to give the EDM-DEDM procedure which, when applied to practical molecular-replacement cases, was able to improve the model structures. In this paper, it is shown that EDM-DEDM suffers from some critical points that did not allow cyclical application of the procedure. These points are identified and modifications are made to allow iteration of the procedure. The applications indicate that EDM-DEDM may become a fundamental tool in protein crystallography.