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["Zhenhua Zeng" <myid520@163.com>]

I think the problem come from your ngkpt.
You should use a k-point grid as less 8x8x8 more dense.

Best Wishes                      
Zhenhua Zeng

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From：jedokim
sent:：2008-11-11 11:13:49
To：forum@abinit.org
Subject：[abinit-forum] Negative phonon energy for CaF2

>
>
>Hi all,
>I am trying to calculate the phonon band structure using rft. I have  
>tried the optimization of the structure and recaculated the phonon  
>dispersion and still getting negative values. I am at a loss. It may  
>be I'm doing something fundamently wrong. Can anyone find anything  
>wrong with the below input file? Caf_2 has an fm2m symitry FCC  
>structure where primitive cell has Ca at 000 and F at 0.25 0.25 0.25  
>and 0.75 0.75 0.75.
>
>
># 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*10.4422        # This is equivalent to   10.61 10.61 10.61
>     rprim    0.0  0.5  0.5
>              0.5  0.5  0.5
>              0.5  0.5  0.5
>#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).
>
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