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["Lee Geunsik" <maxgeun@postech.ac.kr>]

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.