Low-resolution phases: influence on SIR syntheses and retrieval with double-step filtration

Different sources of error in single isomorphous replacement (SIR) maps are analysed. It is shown that SIR maps are good enough in themselves to localize a molecule but the absence of low-resolution reflections can distort the image of the molecule. Even in this case the molecule can be reliably localized using the double-step filtration technique of Urzhumtsev, Lunin & Luzyanina [Acta Cryst. (1989), A45, 34-39] if parameters are chosen appropriately. On the other hand, such syntheses may be considerably improved by addition of low-resolution structure factors. A very simple procedure is suggested to retrieve unknown phases for these structure factors.