Get systematic absences from .sca file
Scalepack outputs only systematic absences, and not the rest of reflections. The best way to identify screw axis is probably to look at the whole set of reflections, so that you can compare systematic absences with present reflections. Another relevant situation is when you need to identify which axis are screw in, say, P2221. You have to run scalepack at least twice to do it.
So, the general rule is:
1. Run scalepack in the spacegroup with no screw axis (e.g. P2, P222, I422, etc).
2. Use the following python script to extract specific reflection types.
The syntax is
./gethkl <file-name> 'absence code'
Absence code should be in quotes but has a human(e) format, such as (0,0,l), (0,2k,0), etc. Other examples are (h,0,0), (h,1,2), (h,2k,4) and so on.
Enjoy. (Tested on Ubuntu Gutsy, hence python 2.5).
Click here for the code [1]