>There does indeed appear to be a tendancy for the SHELXC values to become slightly negative at the high resolution end, i.e. when a value of zero would be expected for pure noise, so maybe it is something more fundamental?!
I think it is something more fundamental, and I've just explored whether having random phase shifts in Fanom of progressively-increasing magnitude would do this, but it seems not.
Could also be the way of selecting individual measurements of Dano1 and Dano2 from each Bijvoet pair--perhaps some sort of systematic correction factor?
As for a bug, it could be that when calculating the two sets Dano1 and Dano2, the sign of one might be somehow getting flipped, which would yield negative CCanoms. But then CCanom would never be positive, so that's out.
JPK
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