I seem to recall this being discussed at some point. For the difference electron density map, there clearly isn't a downside to loss of reflections, i.e., the coefficients in the map generation are formally zero for Fo-Fc (which all the scaling, weight, sigma-A bits in there). If the phases are fairly good, then you are assuming that the Fobs agrees perfectly with the Fcalc. You don't gain any new details in the map, but the map isn't distorted with loss of these terms.
As for the 2Fo-Fc (2mFo-DFc, or something like that) electron density map, it again assumes that the phases are in good shape, and you essentially lose any new information you could gain from the addition of new Fobs terms, but the map isn't distorted since the terms are zero. I seem to recall in the dark ages that you could make an Fobs map, and included Fcalc, or some fraction of Fcalc (like 0.5Fc) in as the Fobs term for missing reflections. That way the amplitudes are closer to being correct for resolution shells that are fairly incomplete. Anyone else remember this for small molecule structures? The real issue, generally, is that the phases are the most important factor in making a good map, and the structure factors are, unfortunately, weaker contributors to features in the maps. Bernie On Fri, March 23, 2007 4:02 am, Eleanor Dodson wrote: > This is a good point - I had thought that D would be very low for an > incomplete shell, but that doesnt seem to be true.. > > Garib - what do you think? > Eleanor > > > Petrus H Zwart wrote: >>> I typically process my data to a maximum I/sig near 1, and >>> completeness in >>> the highest resolution shell to 50% or greater. It >>> >> >> What about maps computed of very incomplete datasets at high resolution? >> Don't you get a false sense of details when the missing reflections are >> filled in with DFc when computing a 2MFo-DFc map? >> >> P >> >> >> >
