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[Katalin Gaal-Nagy <katalin.gaal-nagy@physik.uni-regensburg.de>]

Dear Lee Geunsik,

I was investigating in the high-pressure phase transitions in Si time ago. 
From 
my results for the sequence beta-tin->Imma->sh I found no soft-modes for the 
beta-tin structure even for very high pressures meaning small volumes, if I 
just 
concern the bct lattice. This work is still in progress and hopefully I will 
publish my results soon. Besides I was doing this calculations with pwscf, 
not 
with abinit, but I guess the code doesn't matter.

Best wishes,
Katalin


> 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.
> 
>