[Pw_forum] "q mesh breaks symmetry" error even with q mesh being the same as k mesh

[Thomas Brumme]

Dear all,

I want to calculate the phonon frequencies on a regular q mesh specified 
by using
ldisp=.true. and the nq1, nq2, nq3 variables. Yet, I always get the error

q-mesh breaks symmetry

even if the grid has the same dimensions as the k mesh in the PWscf 
calculation.
This is weird as the q mesh is exactly the same as the k mesh, which was 
created
by applying the symmetries to a regular grid. The system is LCO and the 
input for
PWscf (in the reduced cell) is:

&control
  calculation   = 'scf',
  restart_mode  = 'from_scratch',
  prefix        = 'LCO',
  pseudo_dir    = './',
  outdir        = './tmp/',
  nstep         = 300,
  wf_collect    = .true.,
/
&system
  ibrav       = 0,
  nat         = 14,
  ntyp        = 3,
  ecutwfc     = 200,
  occupations = 'smearing',
  smearing    = 'gauss',
  degauss     = 0.01,
  nspin       = 2,
  starting_magnetization(3) = 0.02,
  nr1 = 128,
  nr2 = 128,
  nr3 = 128,
/
&electrons
  electron_maxstep = 250,
  diagonalization  = 'cg',
  conv_thr         = 1.0d-10,
/
CELL_PARAMETERS (angstrom)
    5.261112503   0.000000000   0.000000000
    0.000000000   5.330947322   0.000000000
   -2.630556252  -0.000000000   6.548827231
ATOMIC_SPECIES
  La   138.90547   La1.UPF
  O    15.999   O.pz-n-mt.UPF
  Cu  63.546   Cu1.UPF
K_POINTS automatic
  10 10 6 0 0 0
ATOMIC_POSITIONS (crystal)
...

I defined the FFT grid by hand since then the code does not drop the 
symmetries
including fractional translations... Could this be the problem? Or that 
I did choose
a FFT grid which has the same number of points in x/y/z directions? Or 
could this
be due to a similar problem" as in hexagonal crystals where shifting the 
k mesh
away from Gamma is a bad idea? Or is there an obvious error in my input 
for ph.x:

  &inputph
   tr2_ph   = 1.0d-18,
   prefix   = 'LCO',
   amass(1) = 138.90547,
   amass(2) = 15.999,
   amass(3) = 63.546,
   outdir   = './tmp/',
   fildyn   = 'LCO.dynG',
   ldisp=.true.,
   nq1=10, nq2=10, nq3=6,
   fildvscf = 'LCO.dvscf',
   verbosity = 'high',
  /

I searched the archive however I couldn't find a solution to my problem. 
Thus, any
help would be very much appreciated :)

Regards

Thomas

-- 
Dr. rer. nat. Thomas Brumme
Max Planck Institute for the Structure and Dynamics of Matter
Luruper Chaussee 149
22761 Hamburg

Tel:  +49 (0)40 8998 6557

email: Thomas.Brumme at mpsd.mpg.de