Hello Jakob,
The highest-possible crystal symmetry should always be used, in order to make
the description of the experiment as simple (parsimonious) as possible.
In my book, the R-values you report are the "same" for P21 and P212121 ("same"
meaning "similar enough to be explained by different choice of free
reflections, and different degrees of freedom resulting from having twice as
many atoms"). That means that the higher symmetry is the correct one.
Just note that you can _always_ refine in a lower-symmetry subgroup, without
really changing the scientific insight from the experiment.
P21 is a subgroup of P212121. Note that in your case, you can choose the unique
axis of P21 in three ways - you chose one of them. The other two should give
similar R-values.
You could even refine in P1 (of course with 4 times the number of molecules in
the ASU, compared to P212121, and twice compared to P21), without significantly
changing the R-values.
Hope this helps,
Kay
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