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

Dear jedokim

I think your k-point grid of 2 2 2 may not be dense enough.  Have your done the convergence tests of ground state calculation, e.g. "etotal" with respected to "ecut" and "ngkpt"? If the converged ngkpt is more dense than 2 2 2 then the ecut should become smaller. Also, please check out the nband parameter. For CaF2, I think the input vaue for nband in rf calculation should be 8 because the total number of valence electrons is 16. Anyway, I'm not sure if an extra empty band you added has any contribution on negative phonon frequency.  

Hope this helps.

 

Kind regards,

Thanusit Burinprakhon


On November 12, 2008 6:41:46 PM ICT, "matthieu verstraete" <matthieu.jean.verstraete@gmail.com> wrote:

Yes, and you shouldn't need 100Ha ecut! This is barbaric, almost no point in doing pseudopotentials. Why is it so high?

Matthieu

On Tue, Nov 11, 2008 at 4:23 AM, Zhenhua Zeng <myid520@163.com> wrote:

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

Best Wishes
Zhenhua Zeng

-------------------------------------------------------------
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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-------------------------------------------------------------

--
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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