Hi, My favorite explanation for why differences show at full height in a 2Fo-Fc map is that the map is really a Fc + 2(Fo-Fc) map. You start with an Fc map (with maximal bias to the model) and add twice a difference map to it to bring in the experimental data. The half height features of the difference map are doubled to full high in the computation. The question of why they are only half height in a Fo-Fc map to begin with is a more subtle question. The seat-of-the-pants answer is that the Fo-Fc coefficients only consider the model's failure to match the Fo, not the additional errors in phase. I'm sure there is a paper in Acta Cryst from the 1950's that gives the definitive derivation but I don't know the reference. Dale Tronrud 丁玮 wrote:
Dear all, Why the density map with coeffients |Fobs|-|Fcalc| show the density only at half the height, and with coeffients 2|Fobs|-|Fcalc| show the density at full height? Thanks!
Breth [email protected] 2010-01-15
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