This routine offers a detailed Analysis of the Bijvoet (Friedel) Pairs found in an Fo/Fc-CIF..
Required data are a .cif and a .fcf (including Friedel related reflections) for the non-centrosymmetric structure.
Note For the purpose of this analysis the .fcf should NOT be based on a BASF/TWIN refinement. Flack parameter contributions to F(calc) are incorperated in the .fcf only with a TWIN/BASF refinement ! This is not the case with the default Flack parameter determination.
Example: [Flack x = -0.010(4), abs[F1(calc)**2 - F2(calc)**2] > 4 Sigma]
Calculated Bijvoet differences (Horizontal) are plotted against observed Bijvoet differences (Vertical). Vertical bars indicate one sigma spread.
Only differences above the preset sigma difference (Selected) are displayed out of the total number of Bijvoet pairs.
Plot entries are expected to be located in the upper right (or inversion related lower left) quadrant for the correct absolute structure. Deviating entries (i.e. located in the other two quadrants) are in red.
Reflection indices are displayed for the the largest Bijvoet differences.
Full details can be found in the listing file.
Sigma = sqrt(sigma**2(F1(obs)**2) + sigma**(F2(obs)**2)), where F1 and F2 represent the Friedel (Bijvoet) related reflections.
511 out of the 7738 Friedel related reflection meet the 4*sigma criterium, all except one are found to confirm the selected absolute structure. (note: the figure above is based on the less strickt 0.25*sigma criterium)
The Average Ratio parameter is expected to have a value close to 1.0 for a strongly determined absolute structure and is defined as:
Sum(weight((Fo1**2-Fo2**2)/(Fc1**2-Fc2**2))) / Sum(weight)
with: weight = abs(Fc1**2-Fc2**2) / sigma
Note: The strongest Bijvoet differences are shown with reflection indices.
An alternative analysis of the absolute structure is provided under the heading Bayesian Statistics.
Three types of analysis are done:
The value of FLEQ (SU) is to be compaired with the value of FLACK(SU).
P2(true) gives the probability (scale 0 to 1) that the current absolute structure is the correct one, assuming that the compound is enantiopure.
Note that the MoKa diffraction data used here are for the Nonius Ammonium bi tartrate test crystal of known absolute configuration. The Friedel pair coverage is 99.8 %.
Concluding remark: It is advised to base the analysis on data with close to 100% Friedel coverage and a fully refined structure. Having said that, it turns out that, in our experience, an analysis in the early isotropic refinement stage already predicts the correct absolute structure.