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I believe some, may not be all, negative density may be due to the choice of wavelength. C,O and N are not so sensitive to normally used wavelength (1-1.54A), but S does. This is more true when you have higher resolution and better quality data. You can estimate this by f'. By the way, although I am not a real supporter of "radicals from damage", I believe that a photo-induced (here X-ray) chemical reaction is much different from a heat-induced one. Low temperature is not a problem. But this not for diffusion, as it is ~100% heat- related. Please point out if I am wrong. > This "assignment" of free radicals to damage is often made > (flippantly) in the literature, but I feel a strong need to point > out that there is NO EVIDENCE of a free radical diffusion mechanism > for radiation damage below ~130K. Lijun Liu, PhD Institute of Molecular Biology HHMI & Department of Physics University of Oregon Eugene, OR 97403 541-346-4080