[Pw_forum] Problems with phonones in Fe

[Иван Булдашев]

Dear QE users,
I studied the thermodynamical parameters of bcc Fe, so I need to obtain 
the phonon frequencies, but there are some problems with ph.x.
I normally performed scf calculations after convergence tests and got 
electronic structure. But phonon calculations worked very long. For 
instance, look at this input (two atoms of Fe, simple cubic, nbnd=24):

phonons of Fe
  &inputph
   tr2_ph=1.e-10,
   alpha_mix=0.1,
   ldisp=.true.,
   nq1=8, nq2=8, nq3=8
   amass(1)=55.845,
   outdir='/home/buldashev/temp9',
   fildyn='fe.dyn',
  /

This calculation took more than five days on 48 processor cores.
When I try to compute 16 atoms of Fe, it worked incredibly long and 
spent more than two days for one q-point.
Why it work so long and how I can calculate phonones for big system (16 
or more atoms)?

The second problem seems very strange. I got the positive frequencies 
(for system with two Fe) in reciprocal-space:

  Diagonalizing the dynamical matrix
      q = (    0.000000000   0.000000000   0.000000000 )
  **************************************************************************
      omega( 1) =       7.991400 [THz] =     266.564426 [cm-1]
      omega( 2) =       7.991400 [THz] =     266.564426 [cm-1]
      omega( 3) =       7.991400 [THz] =     266.564426 [cm-1]
      omega( 4) =      16.792406 [THz] =     560.134373 [cm-1]
      omega( 5) =      16.792406 [THz] =     560.134373 [cm-1]
      omega( 6) =      16.792406 [THz] =     560.134373 [cm-1]
  **************************************************************************

but after matdyn.x they become negative:

q =       0.0000      0.0000      0.0000
  **************************************************************************
      omega( 1) =     -14.768968 [THz] =    -492.639743 [cm-1]
  (  0.130412   0.000000    -0.554126   0.000000     0.419449 0.000000   )
  ( -0.130412   0.000000     0.554126   0.000000    -0.419449 0.000000   )

Why they change their signs and how I can improve it?

Thank you.
Buldashev Ivan, student.
South Ural State University.