The ABINIT Users Mailing List ( CLOSED )

["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