(or .cif).
1.3.6.3 - FCF-VALIDATION
This tool offers validation of
.fcf files (in combination with the associated
.cif) for
completeness and unusual features. Given the files
name.cif and
name.fcf the FCF-
Validation can also be invoked as
platon -V name.cif. The result of the analysis can be
found in
name.ckf Note: The 'data_' names in the
.cif and
.fcf should be identical. More
information can be found in
Chapter 8.
http://www.cryst.chem.uu.nl/spek/platon/FCF-VALIDATION.pdf
.
1.3.6.4 – DifFourier – Peak Search and Analysis of a Difference Density Map
This tool provides an analysis of the (final) difference Fourier map. Required files are a
.cif
and a
.fcf. This function can also be called via
platon -D compound.cif. Density maxima
and minima are listed along with their distances to the four nearest atoms in the model.
1.3.6.5 - Analysis of Variance
This tool analysis Fobs versus Fcalc data. The graphical output consists of a Normal
Probabilty Plot (Abrahams & Keve, 1971), a line plot of Iobs against Icalc and a Log-Log
plot.The listing file reports R-values against resolution.
Sub-menu #0 – (
Section 1.4.32) – Options
1.3.6.6 - Bijvoet-Pair Analysis and Bayesian Statistics
This tool offers a detailed analysis of the Bijvoet (Friedel) pairs found in an Fo/Fc reflection
CIF, both as a Scatter Plot and in terms of Bayesian Statistics, to establish the absolute
structure in terms of the Hooft parameter. Details on the theory behind the Hooft parameter
can be found in Hooft et al. (2008). See also
Rob Hooft's Website on Absolute Structure
Determination."
Required data are a
.cif and an
.fcf (including Friedel (Bijvoet) related reflection pairs) for
the non-centrosymmetric structure. The structure factors that are used in the analysis are by
default re-calculated from the parameter data in the
.cif file. Alternatively, calculated
structure factors can be taken from the
.fcf by setting an appropriate switch (see warning
below!).
When the F(calc) values are chosen to be taken from the
.fcf, that file should NOT be based
on a BASF/TWIN refinement. Flack parameter contributions to F(calc) are incorporated by
SHELXL-97 into the
.fcf with a BASF/TWIN refinement, making them useless for this
application. That is not the case with the default Flack parameter (hole-in-one)
determination.
The currently most used standard procedure for the determination of the absolute structure
with X-ray diffraction techniques is based on the determination of the Flack parameter with
its associated standard uncertainty as part of the least-squares refinement procedure
(preferably with the BASF/TWIN instructions). The alternative post-refinement procedure
the Friedel (Bijvoet) related reflections.
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. A least squares line (green) is calculated through the points. This line
is expected to run from the lower left to the upper right corner for the correct absolute
assignment. A change to the opposite absolute structure is indicated when this line runs
from the upper left to the lower right corner. All except one of the 513 pairs that meet the
4*Sigma criterium confirm the selected absolute structure (given that the compound is
enantiopure). 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
More details can be found in the listing file.
Bayesian Statistics
An alternative analysis of the absolute structure is provided under the heading
Bayesian
Statistics. Three types of analysis are done:
P2:Probability assuming two possiblities only (i.e one of the two possible enantiomorphs)
P3:Probability assuming three possibilities only (i.e. P2 + the racemic twin option).
Hooft:Probability based on a continuum of hypotheses. The displayed parameter is cast in
the form of 'Flack Equivalent' Hooft parameter. The value of Hooft(S.u.) is to be compared
with the value of Flack(s.u.). P2(true) gives the probability (scale 0 to 1) that the current
absolute structure is the correct one, assuming that the compound is enantiopure.
The
Friedel Coverage is defined as the ratio of the number of Friedel pairs in the data set
and the maximum possible number * 100%.
Concluding remarks
I
t is advised to base the analysis on data with close to 100% Friedel coverage and on 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.
The default analysis assumes a Gaussian error distribution. The validity of this assumption
can be tested with a Normal Probability plot (below).
Sub-Menu #0 – (
Section 1.4.29) – Options
1.3.6.7 – ASYM-EXPECT – Exact Expected Reflection Number Count
This tool calculates the exact number of reflections in the asymmetric unit for a given (e.g.
in the CIF) theta-max. In non-centrosymmetric structures both the total number of
reflections in the asymmetric unit (including Friedel/Bijvoet related reflections) and the
number of 'Friedel-averaged' reflections are given. Systematic absences are not included in
these counts. An approximate count as a function of theta is generated with the EXPECT-
HKL function. ASYM-EXPECT is run automatically as part of the PLATON/CALC
instruction and as part of the VALIDATION check.