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[joukj@hrem.stm.tudelft.nl (Jacob (=Jouk) Jansen)]

Dr.N. Choudhury" <dynamics@apsara.barc.ernet.in> wrote on 7-JAN-2005 
12:29:06.93

>I used ABINIT (v433)  for study of a monoclinic perovskite supercell.
>While ABINIT assigns space group Pm for the structure enclosed below,
>FINDSYM (http://physics1.byu.edu/cgi-bin/findsym.cgi ) assigns space group
>Cm. The polarization has [aab] type symmetry (Cartesian) which Vanderbilt
>and Cohen, (Phys. Rev. B.63, 094108 (2001)) assign to Cm symmetry.
>
>Why are the space groups assigned different?
>
>Please note that I have used high precision for the struc generator
>in both ABINIT and FINDSYM, but  FINDSYM  rounds off after 5 decimals
>
>
>Thanks,
>Narayani
>
>Narayani Choudhury
>Solid State Physics Division, Bhabha Atomic Research Centre
>Trombay, Mumbai 400 085, India; email: dynamics@apsara.barc.ernet.in

A quick check reveals to me that FINDSYM is wrong.
The reason is that the C-centering is not present. You see that mos clear
from the only atom with type "2" (atom-numer 4)

>Type of each atom:
>  1  1  1  2  3  3  4  4  4  4  4  4  4  4  4
>Position of each atom (dimensionless coordinates)
>   1  -0.00825  -0.01651   0.02885
>   2   0.33892   0.67784   0.37681
>   3   0.65782   0.31564   0.69559
>   4   0.00413   0.00826   0.50159
>   5   0.33546   0.67092   0.82270
>   6   0.67001   0.34003   0.17816
>   7   0.19553   0.85285   0.65196
>   8   0.18265   0.36531   0.65512
>   9   0.65731   0.85285   0.65196
>  10   0.34035   0.18281   0.30794
>  11   0.84246   0.18281   0.30794
>  12   0.84162   0.68324   0.30316
>  13   0.52033   0.04067  -0.02475
>  14   0.00749   0.53079  -0.01957
>  15   0.52330   0.53079  -0.01957

If C-centering would be present also an atom of type "2" would have been
located at 
    4b   0.50413   0.50826   0.50159
But since there is only one atom of type 2 this is not thae case.

            Jouk Jansen


Bush : All votes are equal but some votes are more equal than others.

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  Jouk Jansen
                 
  joukj@hrem.stm.tudelft.nl

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