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[verstraete@pcpm.ucl.ac.be]

Hello Geunsik,

there have been several mails about this in forum a while ago. The main 
problem is the breaking of exact translational invariance with a certain 
piece of the pseudopotential. Try a different psp. You may also be 
insufficiently converged, but your ecut looks good. Maybe increasing the 
smearing a little, but I doubt it. The hgh pseudos are often the worst for 
convergence properties. Stick to Trouiller Martins.

Matthieu



On Tue, 13 Jul 2004, Lee Geunsik wrote:

> Dear abinitors,
>
> I am looking for unstable phonon modes of beta-tin (body-centered tetragonal) 
> Si.
> Beta-tin is the high pressure phase next to diamond.
> The problem is to test phonon freq. convergence with respect to input 
> parameters.
>
> With the volume corresponding beta-tin phase, electronic calculation shows 
> that it is metalic.
> So I computed phonon freq. for q=(0,0,0) with occopt=3, tsmear=0.004, nband=6.
> Following is relevant input file.
>
> ndtset  6   udtset 3 2
>
> ecut1?   16.0
> ngkpt1?   3*16
>
> ecut2?   16.0
> ngkpt2?   3*18
>
> ecut3?   18.0
> ngkpt3?   3*16
>
> #Ground state
> kptopt?1   1
> tolvrs?1   1.0d-18
> prtden?1   1
> iscf?1     5
>
> getwfk?2  -1
> rfatpol?2  1 2
>  rfdir?2  1 1 1
> rfphon?2  1
>   nqpt?2  1
> kptopt?2  2
> tolvrs?2  1.0d-8
>   iscf?2  5
>  prtwf?2  0
>
> qpt?2  0.0 0.0 0.0
>
> #Common input variables
> occopt 3   tsmear 0.004
> nband 6
>
> fftalg 412
> acell  3*8.88
> rprim  0.0  0.5  0.5
>       0.5  0.0  0.5
>       0.5  0.5  0.0
> ntypat 1
> znucl 14
> natom 2
> typat 1 1
> xred  0.0  0.0  0.0
>      1/4  1/4  1/4
> ixc 2
> nshiftk 1
> shiftk 0.5 0.5 0.5
> nstep 40
> diemac 12.0
>
>
> Following is the results.
>
> ecut=16 Hart.  ngkpt=3*16
> Phonon frequencies in cm-1    :
> -  7.573314E+01  7.699138E+01  7.720520E+01  7.450543E+02  7.451827E+02
> -  7.452047E+02
>
> ecut=16 Hart.  ngkpt=3*18
> Phonon frequencies in cm-1    :
> -  7.572324E+01  7.699129E+01  7.720675E+01  7.448678E+02  7.452402E+02
> -  7.453041E+02
>
> ecut=18 Hart.  ngkpt=3*16
> Phonon frequencies in cm-1    :
> -  7.630890E+01  7.747308E+01  7.767107E+01  7.450455E+02  7.451757E+02
> -  7.451981E+02
>
> All accoustic freq. are quite larger than zero.
> What is the possible reasons?
> Could you give some information?
>
> Thanks a lot.
> Geunsik.

--
===================================================================
Matthieu Verstraete               mailto:verstraete@pcpm.ucl.ac.be 
PCPM, Boltzmann, pl. Croix du Sud, 1            tel: 010/ 47 33 59
B-1348 Louvain-la-Neuve  Belgium                fax: 010/ 47 34 52