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[Mozhgan Amini <mozhganamini@yahoo.com>]

Dear matthieu,

 

Here is my final results for frequencies of PbTiO3 in cubic phase(with no imaginary frequency):

 

  Phonon wavevector (reduced coordinates) :  0.00000  0.00000  0.00000
 Phonon energies in Hartree :
   1.375306E-05  1.375306E-05  1.611297E-05  4.419942E-04  4.419942E-04
   4.421844E-04  9.409520E-04  9.409520E-04  9.409555E-04  9.960169E-04
   9.960169E-04  9.960169E-04  2.658582E-03  2.658628E-03  2.658628E-03
 Phonon frequencies in cm-1    :
-  3.018449E+00  3.018449E+00  3.536389E+00  9.700651E+01  9.700651E+01
-  9.704826E+01  2.065151E+02  2.065151E+02  2.065159E+02  2.186005E+02
-  2.186005E+02  2.186005E+02  5.834912E+02  5.835014E+02  5.835014E+02

 

my input file (rf1):

 
ndtset  3
kptopt1   1               
tolvrs1   1.0d-18      

iscf1   5            
#Response Function calculation : d/dk
rfelfd2   2             

rfdir2   1 1 1                                
rfdir 1 1 1
nqpt2   1
qpt2   0.0 0.0 0.0   
getwfk2   -1            
kptopt2   2         
iscf2  -3             

tolwfr2   1.0d-22      
#Response Function calculation : electric field perturbation and phonons
rfphon3   1        
rfatpol3   1 5      

rfelfd3   3      
rfdir3   1 1 1                     
nqpt3   1
qpt3   0.0 0.0 0.0  
getwfk3   -2           

getddk3   -1            
kptopt3   2            

tolvrs3   1.0d-8
iscf3   5            
#######################################################################
#Common input variables

#Definition of the unit cell
acell    7.7303653285E+00  7.7303653285E+00  7.7303653285E+00 Bohr    
rprim  1.0  0.0  0.0   
       0.0  1.0  0.0   
       0.0  0.0  1.0   
#Definition of the atom types
ntypat 3              

znucl 82 8 22                             
#Definition of the atoms
natom 5               
typat  1 3 2 2 2      
xred   0.0    0.0    0.0
       0.5    0.5    0.5
       0.5    0.5    0.0
       0.5    0.0    0.5
       0.0    0.5    0.5

nband  20               
#Exchange-correlation functional
ixc 1             # LDA Teter Pade parametrization

#Definition of the planewave basis set
ecut    60.0        
#Definition of the k-point grid
kptrlatt  6  0  0     

          0  6  0       

          0  0  6          

#Definition of the SCF procedure
nstep 100         
diemac 6.0     

 

Thanks,          
                      

--- On Mon, 12/8/08, matthieu verstraete <matthieu.jean.verstraete@gmail.com> wrote:

From: matthieu verstraete <matthieu.jean.verstraete@gmail.com>
Subject: Re: [abinit-forum] soft mode phonons
To: forum@abinit.org
Date: Monday, December 8, 2008, 2:13 PM

Hello

Abinit can certainly calculate the soft modes (find publications by Ghosez or Vanderbilt, for some examples). If you don't have any, your calculation is probably not converged:

1) if the electronic temperature is too high (providing you are using one)
2) if the lattice is strained the frequencies can be pushed positive
3) if you don't have enough kpoints or a large enough ecut.
4) if your qpoint grid is not fine enough (or does not contain the qvector where the phonon goes soft) then you will miss the negative mode, depending on the details of the phonon interpolation.

there may be other reasons as well. Are you certain your compound is ferroelectric? Are you just reproducing a tutorial or did you adapt it -

Again, and as usual: if you write to forum you should include all system details and follow  Nettiquette!!   http://www.abinit.org/community/?text=netiquette

If you had done so we could have looked at your input and told you which was the problem, instead of trying to guess!

Matthieu

On Sun, Dec 7, 2008 at 6:43 PM, <mozhganamini@yahoo.com> wrote:

Dear all,

I have done RF1 lesson for one of the ABO3 compounds for getting it's phonon
frequencies and I have gotten some real frequencies. But, I know that this
compound has soft mode in TO1.
Can Abinit code calculates these soft modes or I did some wrong thing in my
calculations?

Thanks alot in advance,
mozhgan amini

--
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dr. Matthieu Verstraete

European Theoretical Spectroscopy Facility (ETSF) 
Dpto. Fisica de Materiales, 
U. del Pais Vasco,            
Centro Joxe Mari Korta, Av. de Tolosa, 72,   Phone: +34-943018393
E-20018 Donostia-San Sebastian, Spain        Fax  : +34-943018390

Mail : matthieu.jean.verstraete@gmail.com
http://www-users.york.ac.uk/~mjv500