TECHNIQUES FOR ABSORPTION CORRECTION

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PLATON implements a variety of established techniques for correction for absorption.

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.

1. Numerical Methods. Supposedly close to exact (1 & 2 based on FACE indexing)
   1. ABST: Analytical following the Alcock version of "de Meulenaer & Tompa"
      This is the method of choise.

   2. ABSG: Gaussian Integration (Modified from Coppens)
      Should give, given a sufficiently fine integration grid, similar results as 'deM&T', but generally more compute intens.

   3. ABSS: Spherical Absorption Correction
      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).

2. Semi-empirical methods (based on redundant/additional experimental data)
   1. ABSP: Psi-Scan data based correction (North et al.)
      Fashionable for Acta Cryst. papers based on serial detector data.

   2. MULABS: Based on multi-scanned reflection data (Blessing).
      Normally used for redundant area (CCD) detector data.

3. Empirical Methods
   1. 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.

The reflection file should be formatted following the SHELX HKLF 4 Format.

h,k,l,F**2,sigma(F**2),N,IX,DX,IY,DY,IZ,DZ

FORMAT:(3I4,2F8.2,I4,6F8.5)

where
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.

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