What does the Addsym Probability output mean?

Subject: Re: Greetings from Melbourne and Addsym query
To: lzc@dl.ac.uk (L. Cranswick)
Date: Mon, 15 Jan 2001 10:56:09 +0100 (MET)
From: spea@xray0.chem.uu.nl (a.l.spek)

> When Addsym says that there is 87% probability of R-3c -
> by what criteria are these percentage points being taken
> away and are these shown in the Platon output?
> (I can't see this in the on-line Addsym manual?)


The percentage issue is related to an observation, first reported
by Dick Marsh, that for various reasons, not all atoms in the
structure fit within the higher symmetry regime. This can happen
easily when a centrosymmetric structure is refined in a non-centro
symmetric spacegroup or with misassigned atomtypes.

ADDSYM therefore allows for a certain percentage of non-fitting
atoms. 

Partial fit is common in cases of pseudo-symmetry. Detailed
inspection should give the final answer.

(From the Dec 2001 Addsym manual, plus some extra parameter numbers)

Optional Keyboard Instructions

where:

• element_name - Search for higher symmetry for specified element only.

• EQUAL - Search with all atom type treated as equivalent.

• EXACT - All atoms should fit for given criteria.

• SHELX - A new shelxl.res file is generated for the proposed space group

• PLOT - A PLUTON-plot of the averaged structure in the proposed space group is generated.

• ang - Angle criterium in search for metrical symmetry of the lattice (default 1.0 degree).

• d1 - Distance criterium for coinciding atoms for non-inversion (pseudo)symmetry elements (default 0.25 Angstrom).

• d2 - Distance criterium for coinciding atoms for (pseudo)inversion and (pseudo) translation symmetry (default 0.45 Angstrom).

Note : By default up to 20 % of the atoms are allowed as mis-fits (i.e. atoms falling out of the default criteria or with missing symmetry related counterparts). The EXACT sub-keyword will put this value to zero. Other values may be set with a SET PAR instruction

(Example: SET PAR 249 40 to set the misfit % to 40).

Here are some more:

PAR(43) - Angle criterium (default 1.00)
PAR(75) - distance criterium (default 0.25)
PAR(76) - distance criterium (default 0.45)