MULABS - Blessing's Method for Absorption Correction

ABSORPTION CORRECTION TOOLS PLATON HOMEPAGE

Related Instruction: MULABS (mu radius (tmin tmax l0max l1max)) (NOCHECK)

See also Absorption Correction Menu

MULABS implements a semi-empirical method for absorption correction using multiple scanned reflections (i.e. multiple symmetry, azimuth equivalent reflection data, CCD-detector datasets) following the excellent algorithm published by Bob Blessing, Acta Cryst (1995), A51, 33-38 (and also available in his SORTAV program).

MULABS as implemented in PLATON requires two files:

  1. name.hkl : A reflection file containing the redundant data set (SHELXL HKLF 4 FORMAT + DIRECTION COSINES)

  2. name.ins : A small pertinent data/instruction file

name.ins should contain the following (free format) data:

TITL ..
CELL lambda a b c alpha beta gamma
SPGR name
MULABS (mu radius (tmin tmax l0max l1max))

Notes:

Generally, only mu and Radius are needed on input.

example:

TITLE test
CELL 0.71073 10.1 10.2 10.3 90 90 90
LATT P C
SYMM -x, y, -z
MULABS 6.5 0.3

MULABS without parameter data gives a correction without the theta dependence (i.e. average transmission value = 1.0). Parameters may be set interactively when MULABS is invoked from the PLATON main menu, with MU (for mu in mm) and RADIUS (mm) instructions.

The MULABS implementation of the Blessing Algoritm is EXPERIMENTAL and may change somewhat on the basis of future experience.

MULABS can also be called from the main PLATON menu (MULABS button). In that case leave out the MULABS line from 'name.ins'

MULABS should be useful in particular for redundant data sets collected with area detectors (e.g. KCCD & SMART). However it can be also useful for redundant datasets originating from serial detector diffractometers.

Note: This implementation is independent from the particular brand/type of diffractometer used; all information needed is included in the (SHELX-style) direction cosines.! (apart from the cell dimensions and Laue symmetry).

A new file (name.hkp) is written (SHELX HKLF 4 style) implementing the absorption correction.

Some additional information is written to compound.lis

The MULABS feature in PLATON may be run from the shell via:

platon name.ins

The required reflection file (SHELX HKLF 4 format) may be obtained using the 'cifin shelx.hkl' utility that comes with the SMN-DENZO distribution for the Nonius KCCD.

Worked Example

See for EXAMPLE DATA.

In general, MULABS will be run as a filter, without any further interaction. However, when the input file does not contain any instructions on the type of absorption correction to be applied or insufficient data, a graphics window/menu will come up.

This section follows an interactive MULABS section for data available below.

Sample data on 'shelx.ins':

TITL S1564A - J.Org.Met.Chem 551(1998)67-79
CELL 0.71073 12.0908 13.6731 17.0346 90 90 90
SPGR PMMM

Invoking PLATON with 'platon shelx.ins' will bring up the PLATON OPENING WINDOW showing the varies options available.
Column 5 lists the available techniques for absorption correction. Fig. 1

MULABS can now be invoked by clicking on 'MULscanABS' giving the absorption correction window/menu ( Fig. 2)

Two messages in RED tell that two more data items will be needed.

The (estimated) radius of the crystal can be supplied via the keyboard by typing 'radius 0.4'. The resulting window looks now like Fig3.

Note that the radius datum has now been accepted (green) and the next data item ready to go (mu 1.5).

The next window (alter entering the mu value) looks like Fig. 4.

The actual calculations are started by clicking on 'next-step'.

A summary of the results of the calculations is shown in Fig.5

The transmission range without (around 1.0) and including spherical correction are given in the lower left corner.

The calculations are terminated by clicking on 'END'.

The corrected data are on shelx.hkp and a more detailed listing on shelx.hkl.

ABSORPTION CORRECTION TOOLS PLATON HOMEPAGE

20-Apr-2001 A.L.Spek