[Pw_forum] ignoring the antiferromagnetic order in the symmetry analysis of phonons

[Lorenzo Paulatto]

On 09/28/2014 11:23 PM, Alaska Subedi wrote:
>   So it would be
> nice to do the symmetry analysis of the phonons by ignoring the
> antiferromagnetic order.
>

Dear Alaska,

there is something you could try, I do not know if it is enough for your 
needs.

First of all you need to do a normal phonon calculation, with the 
antiferro symmetry. Then  you take the dynamical matrix files and you 
edit them by hand, setting the two inequivalent Mn species to the same. 
E.g. let's say your header is like this:

Dynamical matrix file

   2    2  2  7.7249000  0.0000000  0.0000000  0.0000000  0.0000000 
0.0000000
            1  'Mn1 '    96995.8962477164
            2  'Mn2'     96995.8962477164
     1    1      0.0000000      0.0000000      0.0000000
     2    2     -0.5000000      0.5000000      0.5000000

You change it to:

   2    2  2  7.7249000  0.0000000  0.0000000  0.0000000  0.0000000 
0.0000000
            1  'Mn1 '    96995.8962477164
            2  'Mn2'     96995.8962477164
     1    1      0.0000000      0.0000000      0.0000000
     2 *1*     -0.5000000 0.5000000      0.5000000

I.e. you set /all/ the atoms of type Mn2 to be of type Mn1. No need to 
completely remove the Mn2 specie, it only make things more complicated.

Then you resymmetrize the file with the q2qstar.x program from the 
phonon suite:
   q2qstar.x input.dyn output.dyn
The new file will have the symmetry of the non-magnetic system.

I do not guarantee that it makes sense, but it may be enough for your 
application.

kind regards

-- 
Dr. Lorenzo Paulatto
IdR @ IMPMC -- CNRS & Université Paris 6
+33 (0)1 44 275 084 / skype: paulatz
http://www-int.impmc.upmc.fr/~paulatto/
23-24/4é16 Boîte courrier 115, 4 place Jussieu 75252 Paris Cédex 05

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