[Pw_forum] wrong mode symmetry in phonon calculations

[Magdalena Birowska]

Dear all;

I am trying to do phonon calculations for the bulk BP (crystal cell
contains 8 atoms)
space group Cmce (No. 64), point group D_2h.
>From the symmetry consideration there should be 12  irreducible
representations.

Below I include part of my scf input:
 &SYSTEM
   ibrav = 0 ,
   A = 1 ,
CELL_PARAMETERS {alat}
     3.2982000000       0.0000000000       0.0000000000
     0.0000000000      10.1197000000       0.0000000000
     0.0000000000       0.0000000000       4.1209000000
ATOMIC_SPECIES
 P  30.97 P.pz-hgh.UPF
ATOMIC_POSITIONS {crystal}
P       -0.000000000   0.106448467   0.071443342
P        0.500000000   0.893551533   0.571443372
P       -0.000000000   0.893551533   0.928556658
P        0.500000000   0.106448467   0.428556628
P        0.500000000   0.606447305   0.071444660
P        0.000000000   0.393552695   0.571444694
P        0.500000000   0.393552695   0.928555340
P        0.000000000   0.606447305   0.428555306
K_POINTS automatic
 15 5 12 0 0 0

Unfortunatly, the code sees different symmetry modes C_2h with 24
irreducible representations, instead of D_2h.
I checked that QE during the structure relaxation does not break the
symmetry of the crystal
- both input and output structure have the same proper symmetry (No. 64)
(I checked it by used of the Bilbao crystallographic server
http://www.cryst.ehu.es/).
Thus, the phonon code should read the proper one as well.

Below I include part of my phonon output:
     Mode symmetry, C_2h (2/m)  point group:

     freq (  1 -  1) =         -5.4  [cm-1]   --> B_u             I
     freq (  2 -  2) =         -3.2  [cm-1]   --> B_u             I
     freq (  3 -  3) =          8.5  [cm-1]   --> A_u             I
     freq (  4 -  4) =         40.5  [cm-1]   --> B_u             I
     freq (  5 -  5) =         52.0  [cm-1]   --> A_u             I
     freq (  6 -  6) =         95.5  [cm-1]   --> B_u             I
     freq (  7 -  7) =        101.7  [cm-1]   --> B_u             I
     freq (  8 -  8) =        152.3  [cm-1]   --> B_u             I
     freq (  9 -  9) =        179.2  [cm-1]   --> B_g             R
     freq ( 10 - 10) =        182.3  [cm-1]   --> B_g             R
     freq ( 11 - 11) =        202.4  [cm-1]   --> A_g             R
     freq ( 12 - 12) =        216.9  [cm-1]   --> A_g             R
     freq ( 13 - 13) =        361.7  [cm-1]   --> A_g             R
     freq ( 14 - 14) =        363.2  [cm-1]   --> A_g             R
     freq ( 15 - 15) =        392.8  [cm-1]   --> A_g             R
     freq ( 16 - 16) =        396.6  [cm-1]   --> A_g             R
     freq ( 17 - 17) =        415.3  [cm-1]   --> A_u             I
     freq ( 18 - 18) =        420.2  [cm-1]   --> B_g             R
     freq ( 19 - 19) =        420.3  [cm-1]   --> A_u             I
     freq ( 20 - 20) =        421.2  [cm-1]   --> B_g             R
     freq ( 21 - 21) =        432.5  [cm-1]   --> A_g             R
     freq ( 22 - 22) =        448.3  [cm-1]   --> A_g             R
     freq ( 23 - 23) =        458.1  [cm-1]   --> B_u             I
     freq ( 24 - 24) =        458.6  [cm-1]   --> B_u             I

Do you have any suggestions what is the problem?
Many thanks in advance.

Best regards
Magda Birowska
-- 

*Magdalena Popielska (Birowska), PhD*


*Faculty of Physics*


*University of Warsaw*

*ul. Pasteura 5, PL-02-093 Warszawa room no. 5.02*

*Phone: 22 55 32902 (Office)*
*Email: Magdalena.Birowska at fuw.edu.pl <Magdalena.Birowska at fuw.edu.pl>*
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