The quotation marks are used, because it is not entirely obvious to me which kind of properties make one data set superior over another. So the question is to you: What are the "best" charge density data sets available in publications or data banks in that sense that you would like to produce (or did produce) such a set? Why do you think it is a good data set? For example: are the data sets published by Zhurov in Acta Cryst A 67 (2011) with the title “Importance of the consideration of anharmonic motion in charge-density studies: a comparison of variable-temperature studies on two explosives, RDX and HMX” or the one published by Destro (“Experimental Charge Density of alpha-Glycine at 23 K”, J. Phys. Chem. A 104 2000, 1047-1054) qualitatively high? If so, why? If not, why not? For example is a very low R-value or a low Goodness of Fit indicative of a good data set? If so, why? Do you now higher quality data sets? In this question it is as important why one considers a data set to be qualitatively high, not only which data sets are on a qualitatively high standard. I would like to know the best charge density data sets (including Icalc!) currently world wide available and why these are considered to be good.
Dear Julian
I guess that agreement between Fcalc and Fobs is a good indicator, but not sufficient. We must validate the data and models in other ways as well. For example, the ADPs can be validated against neutron diffraction data, and also agreement with high-level periodic ab-initio calculations could be an indication of good quality.
Suggestion for a good dataset: Urea 123 X-ray data (Birkedal et al). These data show a very nice agreement between ADPs from neutron and X-ray which is a good sign. Also the R-values are small, as are the residual density maps.
What is a good dataset? It's a dataset that allows you to answer the questions you address - and perhaps even show you something you did not look for.
Best regards
Anders
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