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

[stefano de gironcoli]

I think that given your Bravais lattice nk3 must be equal to nk1
so
10 nk2 10 0 0 0
or
6 nk2  6 0 0 0
should be fine

10 nk2 6 0 0 0
is not

the automatic unfolding of a grid generate additional points but this 
does not mean they form a regular grid. think about what happens for an 
hexagonal lattice with a shifted grid.

stefano

On 08/08/2016 17:51, Thomas Brumme wrote:
> 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
>