Bijvoet-Pair Analysis


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 !

Example of a Heavy Atom Structure in P1

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.

Example: [Flack x = -0.010(4), abs[F1(calc)**2 - F2(calc)**2] > 4 Sigma]

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 this criterium, all except one are found to confirm the selected absolute structure.

The Average Ratio parameter is expected to have a value close to 1.0 for a strongly determined absolute structure and is defined as: >P> 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.


Bayesian Statistics

An alternative analysis of the absolute structure is provided under the heading Bayesian Statistics Three types of analysis are done:


Example of a Light Atom Structure in P212121


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27-Feb-2006 A.L.Spek