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[Xavier Gonze <gonze@pcpm.ucl.ac.be>]

Dear Hyello,

It is quite often observed, for phonon calculations of metallic systems,
that one has to use a larger number of bands than the default number,
even if the occupation number has dropped to a rather low level already.
So, could you try to increase nband by hand ?

Best,
Xavier





On 06 Sep 2007, at 08:27, hyello25@gmail.com wrote:

> After the calculation of phonon band structure of MgB2, I got minus  
> phonon frequences at Gamma:
>   Phonon at Gamma, with non-analyticity in the
>   direction (cartesian coordinates)  1.00000  0.00000  0.00000
>  Phonon energies in Hartree :
>   -4.397733E-04 -1.705330E-04  3.009955E-06  1.553580E-03   
> 1.579078E-03
>    1.935614E-03  2.270211E-03  2.329649E-03  3.297240E-03
>  Phonon frequencies in cm-1    :
> - -9.651908E+01 -3.742767E+01  6.606088E-01  3.409713E+02  3.465675E 
> +02
> -  4.248181E+02  4.982537E+02  5.112988E+02  7.236605E+02
>
>   The convergence work has been done before to avoid unstable  
> structure, and some variables of input file:
>
> but another similar work with less stringent condition gave  
> positive phonon frequence and quit good band structure. Different  
> variables of the two files and part of the input file are showed  
> below:
> ----minus job :
>  tolvrs1   1.0d-22
>  tsmear 0.008
> ----positive job:
>  tolvrs1   1.0d-21
>  tsmear 0.03
>
>  Can anyone give me some suggestion? Thanks in advance!
> #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-22      # SCF stopping criterion (modify default)
>   rfphon1   0            # Cancel default
>
> #Q vectors for all datasets
>   nqpt1 1
>   qpt2 0 0 0
>   qpt3 0 0 0
> ....(other qpts with 6x6x6 qpoints)
>
> #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-20    # 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   5.0d-11     # This default is active for sets 3-10
> ###################################################################### 
> #
> #Common input variables
>
> occopt 4
> tsmear 0.008
>
> ....(cell and atom variabls)
>
> #Definition of the planewave basis set
>
>      ecut   55.0           # Maximal kinetic energy cut-off, in  
> Hartree
>      ngkpt  6  6  6
>      iscf   3
>   nshiftk   1             # Use one copy of grid only (default)
>   shiftk   0.0 0.0 0.5