The ABINIT Users Mailing List ( CLOSED )

[ธนูสิทธิ์ บุรินทร์ประโคน <thanusit@kku.ac.th>]

Dear Mozhgan and Abinit users

I have worked around the problem regarding negative phonon frequencies
at gamma of AlAs. Some points may be drawn.

By imposing "rfasr" in response function calculation(phonon
perturbation), the calculated phonon frequencies at gamma become closer
to zero (not absolute zero, though). Results with and without "rfasr"
for AlAs are as follows. (Note that rfasr=1 or rfasr=2 give similar
results) 

  #Results from Response function calculation: 
   
   Without "rfasr"
     Phonon frequencies in cm-1:
     - -2.452686E-02 -1.495186E-02  1.293368E-01  3.552019E+02  3.552019E+02
     -  3.912056E+02

   With "rfasr=1" imposed
     Phonon frequencies in cm-1    :
     - -2.608186E-02 -2.452642E-02 -1.194787E-02  3.552019E+02  3.552019E+02
     -  3.912380E+02

When the "asr=1" is re-imposed in running "anaddb" program, (I guess
this is what suggested earlier by Matthieu to "re-inforcing the acoustic
some rule"), the zero phonon frequencies are then obtained. Results is
given below.   

  # Results from Anaddb:

    Phonon frequencies in cm-1    :
   -  0.000000E+00  0.000000E+00  0.000000E+00  3.552019E+02  3.552019E+02
   -  3.912380E+02

The above procedure works for cases in which the negative phonon
frequencies are a results of the breaking of the acoustic sum rule by
the introduction of non strictly translationally invariant of real space
grid. This is the common effect of pseudopotential calculations on
acoustic phonon frequencies and may vary for different psp files or
k-grid density used. Nevertheless, the deviations of the frequencies
from zero are small and can be fixed by the actions of the "rfasr" and
"asr". Hope I get this right.

From the earlier posts in the forum involving this issue, there also
appears the cases in which the negative phonon frequencies are
considerably large as a result of an unstable structure. These are true
features of a crystal. If this the case, imposing "rfasr" and "asr" may
not be a sensible solution.

Hope this helps

Kind regards,
Thanusit Burinprakhon

List of input files (Sorry if too lengthy)

Response function calculation:
(psp files:13al.981214.fhi, 33-As.LDA.fhi)
------------------------------------------------------------------------------

ndtset  3

#Ground state calculation
kptopt1  1             # Automatic generation of k points, taking
                            # into account the symmetry
tolvrs1  1.0d-18       # SCF stopping criterion
iscf1    5             # Self-consistent calculation, using algorithm 5

#Response Function calculation : d/dk
rfelfd2  2                # Activate the calculation of the d/dk
perturbation
rfdir2   1 1 1           # consider the perturbation in the x-,y-, and
z- direction 
                       
nqpt2     1               # Number of q point to calculate phonon frequency
qpt2      0.0 0.0 0.0   # This is a calculation at the Gamma point
getwfk2  -1              # Uses as input the output wf of the previous
dataset
kptopt2   2               # Automatic generation of k points,
                                 # using only the time-reversal symmetry
to decrease
                                # the size of the k point set.

iscf2    -3              # The d/dk perturbation must be treated 
                             # in a non-self-consistent way
tolwfr2   1.0d-22       # Must use tolwfr for non-self-consistent
calculations
                        # Here, the value of tolwfr is very low.

#Response Function calculation : electric field perturbation and phonons
rfphon3    1            # Activate the calculation of the atomic
dispacement perturbations
rfatpol3   1 2          # All the atoms will be displaced
rfelfd3    3              # Activate the calculation of the electric
field perturbation
rfdir3     1 1 1         # All directions are selected
rfasr3     1              #Imposing the acoustic sum rule

nqpt3      1
qpt3       0.0 0.0 0.0   # This is a calculation at the Gamma point

getwfk3   -2         # Uses as input wfs the output wfs of the dataset 1
getddk3   -1        # Uses as input ddk wfs the output of the dataset 2
kptopt3    2          # Automatic generation of k points,
                            # using only the time-reversal symmetry to
decrease
                            # the size of the k point set.
iscf3     5             # Self-consistent calculation, using algorithm 5
tolvrs3   1.0d-8

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

#Definition of the unit cell
acell   3*10.62340      # Converged value structure optimization
rprim   0.0  0.5  0.5     # fcc primitive vectors 
           0.5  0.0  0.5       
           0.5  0.5  0.0       

#Definition of the atom types
ntypat   2         # There are two types of atom
znucl   13 33     # The keyword "znucl" refers to the atomic number of the 
                      # possible type(s) of atom. The pseudopotential(s) 
                      # mentioned in the "files" file must correspond
                      # to the type(s) of atom. Here, type 1 is the
Aluminum,
                      # type 2 is the Arsenic.

#Definition of the atoms
natom   2         # There are two atoms
typat   1 2       # The first is of type 1 (Al), the second is of type 2
(As).
                    
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                     # LDA Teter Pade parametrization

#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               # Use one copy of grid only (default)
shiftk   0.0 0.0 0.5    # This gives the usual fcc Monkhorst-Pack grid
           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         # Maximal number of SCF cycles
diemac  9.0      
prtvol  1
=============================================
=============================================

My input file for "anaddb":
-------------------------------------------------------------------------------
#Flags

ifcflag   1     ! Interatomic force constant flag

#Wavevector grid number 1 (coarse grid, from DDB)

brav    2         ! Bravais Lattice : 1-S.C., 2-F.C., 3-B.C., 4-Hex.)
ngqpt   6  6  6   ! Monkhorst-Pack indices
nqshft  1         ! number of q-points in repeated basic q-cell
q1shft  0.0 0.0 0.0

#Effective charges
asr   1        ! Acoustic Sum Rule. 1 => imposed asymetrically
chneut   1     ! Charge neutrality requirement for effective charges.

#Interatomic force constant info
dipdip  1      ! Dipole-dipole interaction treatment

#Phonon band structure output for band2eps - See note near end for
# dealing with gamma LO-TO splitting issue.
eivec  4

#Wavevector list number 1 (Reduced coordinates and normalization factor)
        
nph1l    71      ! number of phonons in list 1                             
qph1l   (lists of k-points)

#Wavevector list number 2 (Cartesian directions for non-analytic gamma
phonons)

nph2l    1       ! number of directions in list 2

qph2l   1.0  0.0  0.0    0.0

# This line added when defaults were changed (v5.3) to keep the
previous, old 
# behaviour
symdynmat 0
==========================================================



> On October 13, 2008 2:47:18 AM ICT, mozhganamini@yahoo.com wrote:
> 
> Dear all,
> 
> I have done response function1 for PbTiO3 and at the end some of my
acoustic
> phonon frequencies were negative. In spite of the ferroelectricity of
PbTiO3,
> is it a reasonable thing(soft mode)? If not, what should I do? can I
solve it
> maybe by increasing the number of nband?
> 
> Thanks alot in advance
> 
> Best Regards,
> Mozhgan Amini