Frank, This has features in common with my proposal at the 2011 CCP4SW to use <rho_obs / SU(rho_diff)> as a model reliability (or precision) metric (now implemented in EDSTATS). This should not be confused with my proposed accuracy metric which uses a chi-square test based on the values of rho_diff / SU(rho_diff).
The main difference between the precision metric proposed in this paper (i.e. presumably rho_obs / SU(rho_obs)) and mine is that I use SU(rho_diff) in place of SU(rho_obs) because it's much easier to get the former from a QQ plot of rho_diff (making the not unreasonable assumption that if only random errors are present then rho_diff has a normal error distribution). I also tried to compute SU(rho_obs) directly but the results I got didn't look right so I didn't pursue it. Of course dividing rho_obs by the SU is the easy bit: determining the SU values in the first place is the hard part (as I said, I've tried!), and I guess that is what this paper is all about. If they have a good way of getting SU(rho_obs) by FFT then that's great - I can use it in EDSTATS! Ideally I think COOT should implement the whole package of e.d. metrics (RSR, RSCC as well as the ones described above) so people can decide for themselves what works best. I haven't actually been able to read their paper yet so I can't answer these questions. I have to go through our individual reprint ordering service which takes 4-5 days to get it, so hopefully I'll have more info next week. Cheers -- Ian On 14 June 2012 19:34, Frank von Delft <[email protected]> wrote: > Proposition: what the paper below describes (map variance) would be rather > useful to access routinely (e.g. in coot etc), given that it describes how > to calculate by FFT. > > Discuss. (No, I've not read any of it carefully.) > > > > > Acta Cryst. (2012). A68, 464-469 [ doi:10.1107/S0108767312015772 ] > > A practical study of the electron-density-map variance > > C. Giacovazzo and A. Mazzone > > An algorithm is described for the calculation of the variance of the > electron density via fast Fourier transform. The main features of the ratio > electron density/standard deviation are described; it is shown that such a > ratio may be considered a useful criterion for estimating, on an absolute > scale, the quality of an electron-density map, no matter the quality of the > model and the data resolution. In some way, it is a good estimate of the > confidence one should have in the reliability of the proposed structure. The > ratio was tested both for observed and for difference Fourier syntheses, in > order to control the usefulness of the criterion for the most popular maps > in crystallography.
