[Pw_forum] Help, about the phonon calculation of GaN.

[于海林]

Dear all,
I have calculated the phonon of GaN. Firstly, I have performed a relaxation  and attained a good optimized result as shown below.
 
Forces acting on atoms (Ry/au):
     atom    1 type  2   force =     0.00000000    0.00000000   -0.00000539
     atom    2 type  2   force =     0.00000000    0.00000000   -0.00000197
     atom    3 type  1   force =     0.00000000   -0.00000000    0.00000498
     atom    4 type  1   force =     0.00000000   -0.00000000    0.00000237
     Total force =     0.000008     Total SCF correction =     0.000001

     entering subroutine stress ...
          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.04
   0.00000001  -0.00000000   0.00000000          0.00     -0.00      0.00
  -0.00000000   0.00000001  -0.00000000         -0.00      0.00     -0.00
   0.00000000   0.00000000   0.00000071          0.00      0.00      0.10

     bfgs converged in   3 scf cycles and   2 bfgs steps
     (criteria: energy <  1.0E-06, force <  1.0E-05)
     End of BFGS Geometry Optimization

Then, I have performed a scf calculation. The stress convergence was also good as shown below.
 
   entering subroutine stress ...
          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.05
   0.00000009  -0.00000000   0.00000000          0.01     -0.00      0.00
  -0.00000000   0.00000009   0.00000000         -0.00      0.01      0.00
   0.00000000  -0.00000000   0.00000078          0.00     -0.00      0.11

Lastly, I have performed the phonon calculation at G point. The results are shown below.
 
 **************************************************************************
     freq (    1) =      -0.861081 [THz] =     -28.722558 [cm-1]
     freq (    2) =      -0.861081 [THz] =     -28.722558 [cm-1]
     freq (    3) =       0.290776 [THz] =       9.699259 [cm-1]
     freq (    4) =       4.004845 [THz] =     133.587259 [cm-1]
     freq (    5) =       4.004845 [THz] =     133.587259 [cm-1]
     freq (    6) =       9.811711 [THz] =     327.283458 [cm-1]
     freq (    7) =      15.564963 [THz] =     519.191266 [cm-1]
     freq (    8) =      16.231642 [THz] =     541.429307 [cm-1]
     freq (    9) =      16.231642 [THz] =     541.429307 [cm-1]
     freq (   10) =      16.510116 [THz] =     550.718195 [cm-1]
     freq (   11) =      16.510116 [THz] =     550.718195 [cm-1]
     freq (   12) =      20.314461 [THz] =     677.617466 [cm-1]
 **************************************************************************
     Mode symmetry, C_3v (3m)   point group:
     freq (  1 -  2) =        -28.7  [cm-1]   --> E    L_3        I+R
     freq (  3 -  3) =          9.7  [cm-1]   --> A_1  L_1        I+R
     freq (  4 -  5) =        133.6  [cm-1]   --> E    L_3        I+R
     freq (  6 -  6) =        327.3  [cm-1]   --> A_1  L_1        I+R
     freq (  7 -  7) =        519.2  [cm-1]   --> A_1  L_1        I+R
     freq (  8 -  9) =        541.4  [cm-1]   --> E    L_3        I+R
     freq ( 10 - 11) =        550.7  [cm-1]   --> E    L_3        I+R
     freq ( 12 - 12) =        677.6  [cm-1]   --> A_1  L_1        I+R

However, I have some questions about these results.
1. The frequences of freq ( 1) ~freq (    3)  should be zero, why there are not?
2. The hexagonal GaN is belong to C_6v (6mm) point group, why our result is C_3v (3m) point group as above shown?
3. For N and Ga elements, there are too many pesedopotential, how to chose the pesedopotentials?
 
The input files and output files in my calculation are attached in attachments.
 
Thank you in advance!


Best Rgards!


Yu Hailin


College of Physics & Electronic Enginerring,
Changshu Institute of Technology
China

 
 
 
 
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