[Pw_forum] Phonon Calculation Trouble

[宋志达]

Dear All:


I used ph.x to calculate the phonon frequencies at GAMMA point of Ta4Pd3Te16 (its structure can be found at http://www.springermaterials.com).  I had relaxed the structure so that all forces was little than 1mRy/bohr. 


The first time I used parameters :
...
 conv_thr=1.0d-7 
ecutwfc=40
ecutrho=200
...
in self consistent calculation and parameters:
...
tr2_ph=1.0d-12,  
ldiag = .true.
start_irr = 1
last_irr = 33
...
/
0 0 0
in phonon calculations. Note that irrep1-irrep22 are Ag and irrep23-irrep33 are Bg (which are raman active), the rest irreducible representations are not raman active.  I got a reasonable results which can be comparable with experiment but with an exception that the lowest frequency was imaginary:
     freq (  1 -  1) =        -28.6  [cm-1]   --> A_g             R  
     freq ( 38 - 38) =         30.0  [cm-1]   --> A_g             R  
     freq ( 39 - 39) =         36.3  [cm-1]   --> B_g             R  
     ...


Therefore I enhanced my precision as:
...
 conv_thr=1.0d-8 
ecutwfc=40
ecutrho=300
...
------------------
...
tr2_ph=1.0d-14,  
ldiag = .true.
start_irr = 1
last_irr = 33
...
/
0 0 0
I expected to get a better result, but actually I got a worse one:
     freq (  1 -  1) =       -504.0  [cm-1]   --> A_g             R  
     freq (  2 -  2) =       -333.9  [cm-1]   --> A_g             R  
     freq (  3 -  3) =       -251.0  [cm-1]   --> A_g             R  
     freq (  4 -  4) =       -140.0  [cm-1]   --> A_g             R  
     freq ( 41 - 41) =         44.2  [cm-1]   --> A_g             R  
......


I completely can't understand why. Can anybody help me ?


Thanks


--
Zhida Song
Institute of Physics
Chinese Academy of Sciences
P.O. Box 603, Beijing 100190, China


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