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["Anglade Pierre-Matthieu" <anglade@gmail.com>]

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.
>
>



-- 
Pierre-Matthieu Anglade