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["Yong Liu" <yongliu.yl@gmail.com>]

Dear Ganesh,

 

     I've calculated the phonon frequencies at gamma point of PbTiO3 with fully relaxed lattice parameters, but in my case the accoustic branches are negative in the original output file and become zero after I imposing ASR. I think 30 cm-1 is not a huge value thun the asr may can handle this problem. In my view the difference between the calculated and experimental elastic constants can be less than 5~7% if you employ the suitable  pseudopotentials. Since I have not experience on the piezoelectric constant, it is hard for me to say.  

     By the way it is very danger to simulate a materials which is no any experimental relsults reported.

 

2008/1/2, P. Ganesh <pganesh@ciw.edu>:

Dear Yong Liu,

Thank you for the response.

An exp. lat. parameter for my system is not known, but it is fully relaxed.   I am aware that anaddb can impose ASR, but I wonder if I should still get such a big value for the accoustic phonons (~30cm-1).  Also, I am interested in getting the piezo. constants and wonder how reliable they are in my calculation.

Thanks again.

Ganesh

Yong Liu wrote:

Dear Ganesh,

     Do you use the experimental lattice constants or theoretical ones? You'd better fully relaxe the unit cell before perform the RF calculations, since the residual stress may introduce noises. And you can use anaddb to obtain the better results by imposing ASR. Aloutgh I am not the expert in your problem, I think the residual criterion is good enough. Moreover you can try tolvrs 1.0e-12 instead of toldfe 1.0e-20 in the GS dataset.

 

2008/1/2, P. Ganesh <pganesh@ciw.edu>:

Dear All,

Wish you all a very Happy New Year.

I  am trying to compute the phonon DOS of a 10 atom primitive cell
(insulator material).  I am using Abinit 5.4.2.

I find that the accoustic phonons at Gamma (the first 3 numbers in the
freq. list) are far away from 'zero'.  They are all around ~30 cm-1.  I
have used a 6x6x6 kpt. mesh and 65 Ha cutoff.  I also use 'E' filed
perturbation to get the Lo-To splitting.  Apart from the common input
the special inputs for my 4 datasets are below.  (The runs are done
separately and not from same input file)  The nband has been set exactly
to the number of occupied bands in all calculations.

Dataset-1: (GS calculation)

toldfe 1.0d-20

Dataset-2: (non-self consistent calculation)

   iscf   -3         # Need this non-self-consistent option for d/dk
kptopt   2          # Modify default to use time-reversal symmetry
tolwfr   1.0d-22    # Use wave function residual criterion instead
getwfk 1
getden 1

Dataset-3: (ddk calculation at G)
nqpt   1
qpt           0.00000000E+00  0.00000000E+00  0.00000000E+00
   iscf   -3         # Need this non-self-consistent option for d/dk
kptopt   2          # Modify default to use time-reversal symmetry
rfphon   0          # Cancel default
rfelfd   2          # Calculate d/dk wave function only
tolwfr   1.0d-22    # Use wave function residual criterion instead
getwfk 1
rfdir 1 1 1

Dataset-4: (phonon perturbation at G)
nqpt   1
qpt           0.00000000E+00  0.00000000E+00  0.00000000E+00
  getwfk   1          # Use GS wave functions from dataset1
  getddk 1
  kptopt   2          # Need full k-point set for finite-Q response
  rfphon   1          # Do phonon response
  rfatpol   1 10        # Treat displacements of all atoms
  rfdir   1 1 1      # Do all directions (symmetry will be used)
  tolvrs   1.0d-22     # This default is active for sets 3-10
  rfelfd 3
  prtwf 0

I would like to know why the phonon freq. are around 30cm-1 and not
zero.  If it is the kpt. or ecut. that I should increase, which one
matters more?  Is my convergence criteria good enough to get phonons ?


Thanks,
Ganesh

--
Best regards.

-------------------------------------------------------------------------------------
Liu, Yong, Ph D
Institute of Inorganic and nonmetal Materials,
Department of Materials Science and Engineering,
Zhejiang University,PR China

-- 
Best regards.

-------------------------------------------------------------------------------------
Liu, Yong, Ph D
Institute of Inorganic and nonmetal Materials,
Department of Materials Science and Engineering,
Zhejiang University,PR China