Hi!

 

I was getting some unexpected results from Addsym (Platon version 230608) so
I ran some tests to try to clear things up. Now I’m confused to the point
where I need help!

 

The test structure I have used is:

Space group P-1

a = 3.5 Å  b = 12.3 Å  c = 4.5  alpha = beta = gamma = 90 deg

 

Al1 in    0.06   0.85   0.75

Al2 in    0.49   0.33   0.77

 

This is pseudo-Cmcm symmetry. True Cmcm positions (0 x ¾ , ½ x ¾) are only
0.24 Å away. 

 

There are three points that are about to drive me nuts:

 

1) Addsym suggests P21/m instead of Cmcm. It has no effect to increase the
criteria parameters d1, d2 and d3 from the default 0.25 Å to 0.5 Å.

 

2) If I move the structure somewhat closer to Cmcm symmetry by putting Al1
in, say, 0.03   0.85   0.75, I get no new space group suggestion at all!
Addsym produces some lines that say 

 

“E: SYMM TRANSLATION n/a: 0.958 .500 0”

 

during the run and writes

 

“Higher (pseudo)symmetry not compatible,(current symmetry retained)”

 

in the lis-file.

 

3) When I move the atoms very close to the Cmcm positions, e.g.

 

Al1 in    0.01   0.84   0.75

Al2 in    0.49   0.33   0.74

 

then Addsym suggests Cmcm if I launch it from the command line. If I use the
graphical interface, it suggests Pmma instead. That happens even if I place
the atoms in ideal Cmcm positions.

 

Are these things user errors or bugs? If they are user errors I would be
very grateful if anybody could explain what I’m doing wrong or point out
something I can read. I’m aware of the Platon/Addsym resources on the ccp14,
but I didn’t find anything there that helped me.

 

I see on the Platon web page that the newest version should be from July 13
2008, but the version I can download is still from June.

 

Best wishes,

Magnus

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