[Pw_forum] Could not find the mode symmetry after phonon frequency calculation of gamma point

[黄志硕]

Dear Quantum Espresso users,

I just finished the phonon calculation at Gamma point of a material with T_h symmetry. But it seems that the code could not determine the mode symmetry as shown in the output file:

###############################################################
     Mode symmetry, T_h (m-3)   point group:

     freq (  1 -  3) =          9.1  [cm-1]   --> T_u             I
     freq (  4 -  6) =         14.5  [cm-1]   --> T_u             I
     freq (  7 -  9) =         48.9  [cm-1]   --> T_g             R
     freq ( 10 - 12) =         81.9  [cm-1]   --> T_g             R
     freq ( 13 - 15) =        100.1  [cm-1]   --> T_u             I
     freq ( 16 - 16) =        254.2  [cm-1]   -->   ?
     freq ( 17 - 17) =        254.2  [cm-1]   -->   ?
     freq ( 18 - 20) =        265.1  [cm-1]   --> T_g             R
     freq ( 21 - 23) =        347.7  [cm-1]   --> T_u             I
     freq ( 24 - 26) =        349.8  [cm-1]   --> T_u             I
     freq ( 27 - 27) =        350.5  [cm-1]   --> A_u
     freq ( 28 - 28) =        394.8  [cm-1]   -->   ?
     freq ( 29 - 29) =        394.8  [cm-1]   -->   ?
     freq ( 30 - 32) =        395.2  [cm-1]   --> T_u             I
     freq ( 33 - 35) =        396.8  [cm-1]   --> T_g             R
     freq ( 36 - 36) =        398.8  [cm-1]   -->   ?
     freq ( 37 - 37) =        398.8  [cm-1]   -->   ?
     freq ( 38 - 40) =        479.4  [cm-1]   --> T_g             R 
…….
…….
###############################################################

As from above, the code could notice the point group is T_h. However, for the two-fold modes (16 and 17, 28 and 29, 36 and 37, actually, all the two-fold modes have the same problem), which seem to be E_g (or E_u) mode, there is no symmetry information. In stead, there is only two “?” marks. 

Any hint for this problem could be very welcome.

Best regards
Zhishuo Huang

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