2-fold Screw Axes. Along b have –x,1/2+y,-z For points on the axis (i.e. 0,y,0) this looks like a translation only. Just like 1-d centered cell Result for 0k0 h=2n systematic presence If the screw does not run through origin the same presences exit because cannot detect the origin shift!.
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h,0,0 0,k,0 0,0,l h,k,0 h,0,k h,k,0 and h,k,l
Data file is called xl.hkl
(Note LINUX is case sensitive)
Find pattern and weight by frequency of known space groups.
This approach can find patterns that are not there—P2/c vs P21/c P212121 vs P21212
Look for weak reflections I/sigma(I)
Use some high cutoff i.e. 10sigma
This at time fails to find absences