[Pw_forum] RE: Pw_forum digest, Vol 1 #775 - 2 msgs

[Stefano de Gironcoli]

is the value of c/a (celldm(3)) used in the scf calculation the same as 
the one used to generate the q-points ?
Is the crystal symmetry actually lower than tetragonal in the case ?
If so you need to compute additional q-points that would be equivalent in
the Bravais lattice but are not in your crystal.
Run your scf calculation with automatic grid
4 4 4 0 0 0
and stop it as soon as k-points are gerenated and see how many are there.
These are the q-points to be used in the phonon calculation.

Maybe you got a message like (i dont remember it exactly)
   " symmetry not allowed due to fractional translation "
this means your FFT grid is incompatible with the fractional
translation of your crystal and the accepted symmetry is lower
than it could.
Adjust nr1,nr2,nr3 in the input so that fractional translations
connect FFT grid points and all symmetry is restored.

stefano

On Thu, 13 Oct 2005, sun jason wrote:
> 
> Dear all,
> 
> when I doing a phonon spectra test of a primitive tetragonal lattice with
> 4*4*4 uniform q grid, the nscf and the phonon calculation is right.
> but when I perform q2r.x, the program stopped, the error message said:
> 
> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>      task #**********
>      from init : error #       197
>       missing q-point(s)!
>  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
> 
> my input file is like this
> 
>  &input
>    nr1=4, nr2=4, nr3=4, zasr=.true., fild='test444.fc'
>  /
>  18
>  test.dynG
>  dyn.0.2500000,0.0000000,0.0000000
>  dyn.0.5000000,0.0000000,0.0000000
>  dyn.0.2500000,0.2500000,0.0000000
>  dyn.0.5000000,0.2500000,0.0000000
>  dyn.0.5000000,0.5000000,0.0000000
>  dyn.0.0000000,0.0000000,0.1745709
>  dyn.0.2500000,0.0000000,0.1745709
>  dyn.0.5000000,0.0000000,0.1745709
>  dyn.0.2500000,0.2500000,0.1745709
>  dyn.0.5000000,0.2500000,0.1745709
>  dyn.0.5000000,0.5000000,0.1745709
>  dyn.0.0000000,0.0000000,0.3491419
>  dyn.0.2500000,0.0000000,0.3491419
>  dyn.0.5000000,0.0000000,0.3491419
>  dyn.0.2500000,0.2500000,0.3491419
>  dyn.0.5000000,0.2500000,0.3491419
>  dyn.0.5000000,0.5000000,0.3491419
> 
> -----------------------------------
> 
> I calculate the phonon spectra of another lattice, just change the
> lattice constants, atomic species and q vector, the spectra is right. so
> what's wrong with it?
> 
> thank you in advance.
> 
> Best,
> 
> 
> ==============================================
> Jian SUN
> Physics Dept. of Nanjing University
> National Lab. of Solid State Microstructures
> 22 Hankou Road, Gulou District
> Nanjing, Jiangsu Province
> 210093
> China
> ==============================================
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