The ratio of reflections used in the refinement to refined parameters
(the r/p ratio) is important for ensuring precise atomic and
geometric parameters. Higher r/p ratios generally improve the precision.
It is expected that for centrosymmetric space groups, the r/p ratio should be at least 10:1. This also applies for non-centrosymmetric space groups when an element heavier than chlorine is present, because the heavy atom usually improves the diffracting power of the sample to an extent that makes it worthwhile to collect additional high angle data and thereby obtain higher r/p ratios.
For non-centrosymmetric space groups when only elements lighter than argon are present, it is expected that the r/p ratio should be at least 8:1.
If you are having difficulty obtaining a suitable r/p ratio, consider
Note that if you have measured data to at least sin(theta)/lambda of 0.6 (theta(max) = 25 deg. for Mo radiation; theta(max) = 67 deg. for Cu radiation) and you have used all available unique reflections in the refinement, the reflection/parameter test is not applied, because it is considered that you have measured an appropriate quantity of data and that you have made the best possible use of that data.
Also note that with light atom non-centrosymmetric structures where anomalous dispersion effects are insignificant, it is unwise to attempt to use unmerged Friedel-related reflections simply to boost the r/p ratio.
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