TECHNIQUES FOR ABSORPTION CORRECTION
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PLATON implements a variety of established techniques for correction
All techniques listed below use the (De Facto) SHELX76-style of direction
cosines (see below) for the specification of the primary and secondary
beems through the crystal.
Clicking on each one of the tools listed below will bring up more detailed
menu's and info on those utilities.
- Numerical Methods. Supposedly close to exact (1 & 2 based on FACE indexing)
- ABST: Analytical following
the Alcock version of "de Meulenaer & Tompa"
This is the method of choise.
- ABSG: Gaussian Integration
(Modified from Coppens)
Should give, given a sufficiently fine integration grid, similar
results as 'deM&T', but generally more compute intens.
- ABSS: Spherical Absorption
A 'zero-order' correction by approximating the crystal as a sphere
with suitable radius. Often a cure for non-positive definite Uij's for
inorganic compounds (i.e. small or negative U's).
- Semi-empirical methods (based on redundant/additional experimental data)
- ABSP: Psi-Scan data based
correction (North et al.)
Fashionable for Acta Cryst. papers based on serial detector data.
- MULABS: Based on
multi-scanned reflection data (Blessing).
Normally used for redundant
area (CCD) detector data.
- Empirical Methods
- DELABS: Modified and extended implementation of the DIFABS
algorithm (Walker & Stuart)
In general effective but not universially accepted technique.
Additional features provided are:
- ABSX: Comparison of calculated
(i.e. Face-Indexed Alcock) and experimental psi-scans.
- XTAL HABIT for the display
of a face indexed crystal.
Reflection File and Direction Cosines
The reflection file should be formatted following the SHELX HKLF 4
N is the batch number and
IX is the direction cosine of the reverse incident beam with the
reciprocal axis a*,
DZ the direction cosine of the diffracted beam with the
reciprocal axis c*.
THe validity of the direction cosines and their consistency with the
cell dimensions provided is checked by default.
Checking can be overruled. However there should be very good reasons
to do so. Common errors are related to inconsistent cell dimensions
and wrong wavelength.
Absorption correction related Menu instructions
Absorption Correction References
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