[Pw_forum] Phonon Calculation Trouble
宋志达
song.zhida at iphy.ac.cn
Sat Dec 27 12:09:49 CET 2014
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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