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[Jedo <jedokim@umich.edu>]

Hi there,

May be that is the reason.
If I optimize the structure, what will change besides the lattice constant?
Thanks for your help.


Jedo


Anglade Pierre-Matthieu wrote:
> Hi,
> Have you optimised your cell before computing the phonons ? It looks
> like you didn't. This could explain some negative phonons frequency.
>
> regards
> PMA
>
> On Sat, Nov 1, 2008 at 2:21 AM,  <jedokim@umich.edu> wrote:
>   
> > Hi all,
> >
> > I'm a new user of abinit and have some questions on my CaF2 phonon
> > calculation. I end up with negative phonon frequency when I use response
> > theory to calculate my phonons. Following is my input file and corresponding
> > output file. Can someone help me? I tried everything from increasing the
> > iteration numbers, increasing the cut off and lowering the conversion
> > criteria. Please ignore the comment part.
> > Thanks.
> >
> > Input
> > # Crystalline AlAs : computation of the phonon spectrum
> >
> >   ndtset   5
> > #Set 1 : ground state self-consistency
> >
> >  getwfk1   0            # Cancel default
> >  kptopt1   1            # Automatic generation of k points, taking
> >                         # into account the symmetry
> >    nqpt1   0            # Cancel default
> >  tolvrs1   1.0d-18      # SCF stopping criterion (modify default)
> >  rfphon1   0            # Cancel default
> >
> > #Q vectors for all datasets
> >
> > #Complete set of symmetry-inequivalent qpt chosen to be commensurate
> > # with kpt mesh so that only one set of GS wave functions is needed.
> > #Generated automatically by running GS calculation with kptopt=1,
> > # nshift=0, shiftk=0 0 0 (to include gamma) and taking output kpt set
> > # file as qpt set. Set nstep=1 so only one iteration runs.
> >
> >     nqpt   1            # One qpt for each dataset (only 0 or 1 allowed)
> >                         # This is the default for all datasets and must
> >                         #  be explicitly turned off for dataset 1.
> >
> >     qpt2   0.00000000E+00  0.00000000E+00  0.00000000E+00
> >     qpt3   0.00000000E+00  0.00000000E+00  0.00000000E+00
> >     qpt4   -5.00000000E-01  0.00000000E+00  0.00000000E+00
> >     qpt5   -5.00000000E-01  -5.00000000E-01  0.00000000E+00
> >
> >
> >
> > #Set 2 : Response function calculation of d/dk wave function
> >
> >    iscf2   -3         # Need this non-self-consistent option for d/dk
> >  kptopt2   2          # Modify default to use time-reversal symmetry
> >  rfphon2   0          # Cancel default
> >  rfelfd2   2          # Calculate d/dk wave function only
> >  tolvrs2   0.0        # Cancel default for d/dk
> >  tolwfr2   1.0d-22    # Use wave function residual criterion instead
> >
> > #Set 3 : Response function calculation of Q=0 phonons and electric field
> > pert.
> >
> >  getddk3   2          # d/dk wave functions from last dataset
> >  kptopt3   2          # Modify default to use time-reversal symmetry
> >  rfelfd3   3          # Electric-field perturbation response only
> >
> > #Sets 4-10 : Finite-wave-vector phonon calculations (defaults for all
> > datasets)
> >
> >   getwfk   1          # Use GS wave functions from dataset1
> >   kptopt   3          # Need full k-point set for finite-Q response
> >   rfphon   1          # Do phonon response
> >  rfatpol   1 3        # Treat displacements of all atoms
> >    rfdir   1 1 1      # Do all directions (symmetry will be used)
> >   tolvrs   1.0d-8     # This default is active for sets 3-10
> >
> > #######################################################################
> > #Common input variables
> >
> > #Definition of the unit cell
> >    acell   3*10.307         # This is equivalent to   10.61 10.61 10.61
> >    rprim   0.0   0.5   0.5   # In lessons 1 and 2, these primitive vectors
> >            0.5   0.0   0.5   # (to be scaled by acell) were 1 0 0  0 1 0  0
> > 0 1
> >            0.5   0.5   0.0   # that is, the default.
> >
> > #Definition of the atom types
> >   ntypat   2         # There are two types of atom
> >    znucl   20 9     # 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   3         # There are two atoms
> >    typat   1 2 2       # The first is of type 1 (Ca), the second is of type
> > 2 (F).
> >
> >     xred   0.0  0.0  0.0
> >            0.25 0.25 0.25
> >            0.75 0.75 0.75
> >
> > #Gives the number of band, explicitely (do not take the default)
> >      nband  9
> >
> > #Exchange-correlation functional
> >
> > #     ixc   11             # LDA Teter Pade parametrization
> >
> > #Definition of the planewave basis set
> >
> >     ecut   100.0           # Maximal kinetic energy cut-off, in Hartree
> >
> > #Definition of the k-point grid
> >    ngkpt   2  2  2
> >  nshiftk   4              # Use one copy of grid only (default)
> >   shiftk     0.5 0.5 0.5
> >              0.5 0.0 0.0
> >              0.0 0.5 0.0
> >              0.0 0.0 0.5
> >
> > #Definition of the SCF procedure
> >     iscf   5          # Self-consistent calculation, using algorithm 5
> >    nstep   200         # Maximal number of SCF cycles
> >   diemac   7.0        # Although this is not mandatory, it is worth to
> >                       # precondition the SCF cycle. The model dielectric
> >                       # function used as the standard preconditioner
> >                       # is described in the "dielng" input variable section.
> >                       # The dielectric constant of AlAs is smaller that the
> > one of Si (=12).
> >
> >
> >
> > OUT PUT
> > Data set 5
> >  Phonon wavevector (reduced coordinates) :  0.50000  0.50000  0.00000
> >  Phonon energies in Hartree :
> >  -2.309453E-03 -4.549870E-04  1.278573E-03  1.393728E-03  1.396572E-03
> >   1.845115E-03  2.071170E-03  2.399689E-03  3.303399E-03
> >  Phonon frequencies in cm-1    :
> > - -5.068664E+02 -9.985810E+01  2.806144E+02  3.058879E+02  3.065122E+02
> > -  4.049560E+02  4.545694E+02  5.266708E+02  7.250124E+02
> >
> >
> > It seems that negative phonon start to appear from Data set 4.
> >
> >
> >     
>
>
>
>