The ABINIT Users Mailing List ( CLOSED )

[Thanusit Burinprakhon <thanusit@kku.ac.th>]

Dear Abinit users

I have calculated the phonon frequency (at gamma) of AlAs by following the
steps suggested in the tutorial on Response-Function-1. The Abinit used is
version 5.4.4 (MPI version, prepared for a x86_64_linux). The convergence 
tests
of total energy with respected to ecut and k-point grids and structural
optimization were initially performed. Then, the phonon calculation was 
carried
out using the converged ecut, k-point grid, and lattice parameters as the 
input
parameters. The input file is listed below.

With the psp files similar to those used in the tutorial (13al.981214.fhi for
Al and 33as.pspnc for As), the calculated phonon frequencies are as follows.
------------------------------------------------------------------------
  Phonon at Gamma, with non-analyticity in the
  direction (cartesian coordinates)  0.00000  0.00000  1.00000

 Phonon frequencies in cm-1    :
-  9.968522E-02  1.012068E-01  1.972088E-01  3.617507E+02  3.617507E+02
-  3.959641E+02
-----------------------------------------------------------------------
The two TO and LO values are very reasonable compared to experimental (361
cm^-1 , 402 cm^-1) as well as theoretical (363 cm^-1 , 400 cm^-1) values
mentioned in the tutorial.

Then, I've tried using different psp files for As (use 33-As.LDA.fhi instead 
of
33as.pspnc). The whole calculation process were repeated right from scratch.
The calculated phonon frequencies are as follows.
--------------------------------------------------------------------------
  Phonon at Gamma, with non-analyticity in the
  direction (cartesian coordinates)  0.00000  0.00000  1.00000

 Phonon frequencies in cm-1    :
- -2.452686E-02 -1.495186E-02  1.293368E-01  3.552019E+02  3.552019E+02
-  3.912056E+02
----------------------------------------------------------------------------
The two TO and LO values still looks good. However, the strange things(to me)
is the negative values of the frequency of acoustic phonon. Should we be
curious about this (because it indicates that there is something wrong)? or Is
it just a small effect of the psp file used and it is fine as far as the 
values
are close to zero?.

Hope someone can help on this.

Kind regards,
Thanusit Burinprakhon

Physics Department,
Khon Kaen University, Thailand.


My Input file for phonon freq. calculation
----------------------------------------------

ndtset  3

#Ground state calculation
kptopt1  1            
tolvrs1  1.0d-18       
iscf1    5              

#Response Function calculation : d/dk
rfelfd2  2              # Activate the calculation of the d/dk perturbation
rfdir2   1 1 1          
nqpt2     1             
qpt2      0.0 0.0 0.0   # a calculation at the Gamma point
getwfk2  -1            
kptopt2   2            
iscf2    -3            
tolwfr2   1.0d-22

#Response Function calculation : electric field perturbation and phonons
rfphon3    1             
rfatpol3   1 2           
rfelfd3    3             
rfdir3     1 1 1        
nqpt3      1
qpt3       0.0 0.0 0.0   # a calculation at the Gamma point
getwfk3   -2             
getddk3   -1             
kptopt3    2           
iscf3     5            
tolvrs3   1.0d-8

#########################################
#Common input variables

#Definition of the unit cell
acell   3*10.62340     # Converged value from structural optimization
                                     # For 33-As.LDA.fhi psp file, the value 
is
10.62340.
                                     # For 33as.pspnc psp file, the value is
10.585893.
rprim   0.0  0.5  0.5   
            0.5  0.0  0.5   
            0.5  0.5  0.0   

#Definition of the atom types
ntypat   2         
znucl   13 33   

#Definition of the atoms
natom   2        
typat   1 2       
                    
xred   0.0  0.0  0.0
          0.25 0.25 0.25  

#Gives the number of band, explicitely (do not take the default)
nband   4         

#Exchange-correlation functional
ixc   7           

#Definition of the planewave basis set

ecut   50.0      # value giving the convergence of Etot within 0.0001 Hartree

#Definition of the k-point grid
ngkpt    6  6  6 # value giving the convergence of Etot within 0.0001 Hartree
nshiftk  4              
shiftk   0.0 0.0 0.5    
            0.0 0.5 0.0
            0.5 0.0 0.0
           0.5 0.5 0.5

#Definition of the SCF procedure
nstep   50         
diemac  9.0      
prtvol  1
----------------------------------------------------------------------------------------------------------------