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You are probably referring to the following works: Caliandro et al, Acta Cryst. D61 (2005) 556-565 and Caliandro et al, Acta Cryst. D61 (2005) 1080-1087 in which they used density modification to calculate phases for unmeasured reflections, and used the phases to extend the resolution by calculating rough estimates unmeasured amplitudes. Using this technique they actually could improve the electron density. If I'm not mistaken, George Sheldrick has implemented this "Free Lunch" algorithm in SHELXE. /Michel On Fri, 2007-03-23 at 08:05 -0800, Edward Berry wrote: > If instead you allow the missing F's > to "float", calculating them on each cycle from the previous map > using the fillin option, someone has shown (don't have the > reference handy at the moment) that the F's tend toward the true F's > (in the case that they weren't really missing but omitted as part > of the test). > > Ed