[Pw_forum] Wrong frequencies on STO phonons

["Alejandro Rébola"]

Dear Quantum Espresso users,

I'm trying to calculate phonons on cubic SrTiO3 by using ph.x (v.4.3.2). I
relaxed the whole structure (vc-relax) and got reasonable lattice and
internal parameters (experimental 3.905 A, calculated 3.92(PBE)).
Afterwards I performed a scf calculations with this structure and then
used the phonon code. The problem is that I'm obtaining mostly negative
frequencies. I know that cubic STO is unstable, but negative values should
only appear around GAMMA, M and R symmetry points. I've also done this
calculation by using the frozen phonon method and in this case results are
in good agreement with the literature. But the frequencies I got from the
ph.x calculation seem to be completely wrong. I've tried changing
parameters, using a dense k-point grid and a high cutoff, but results are
still wrong. I don't have any clue about what is going wrong, could
anybody help?
I'm copying the input files below. Thank you in advance for your help.
Best,

- Alejandro

The relaxed structure:

 &control
    calculation='scf',
    restart_mode='from_scratch',
    pseudo_dir = '/global/homes/r/rebola/pseudo/',
    outdir='/scratch/scratchdirs/rebola/',
    prefix = 'sto',
 /
 &system
    ibrav=0, celldm(1)=7.46252850, nat=5, ntyp=3,
    ecutwfc=40.0, ecutrho = 400,
 /
 &electrons
    conv_thr =  1.0d-8
    mixing_beta = 0.7
 /
ATOMIC_SPECIES
 Sr 87.62 Sr.pbe-nsp-van.UPF
 Ti 47.867 Ti.pbe-sp-van_ak.UPF
 O  15.9994 O.pbe-van_gipaw.UPF
CELL_PARAMETERS (alat=  7.46252850)
   0.993935603   0.000000000   0.000000000
   0.000000000   0.993935603   0.000000000
   0.000000000   0.000000000   0.993935603
ATOMIC_POSITIONS (alat)
Sr       0.000000000   0.000000000   0.000000000
Ti       0.496967802   0.496967802   0.496967802
O        0.000000000   0.496967802   0.496967802
O        0.496967802   0.000000000   0.496967802
O        0.496967802   0.496967802   0.000000000
K_POINTS (automatic)
4 4 4 0 0 0

The phonon input file:

phonons of STO
 &inputph
  tr2_ph=1.0d-14,
  ldisp=.true.,
  nq1=2, nq2=2, nq3=2
  amass(1)=87.62,
  amass(2)=47.867,
  amass(3)=15.9994,
  epsil=.true.
  prefix='sto',
  outdir='/scratch/scratchdirs/rebola/',
  fildyn='sto.dyn',
 /
~