[Pw_forum] Phonon frequency of Body centred cubic strucure

Merlin Meheut merlin.meheut at lmtg.obs-mip.fr
Sun May 8 16:11:36 CEST 2011

Dear Mayank,

My opinion is that your problem comes from a symetry problem with the 
FFT grid: the FFT grid your system has chosen may not be compatible with 
the symetry of your crystal (which is apparently very symetric). If this 
is the case, yous hould have warnings about symetry operations not taken 
into account in the preamble of your scf output.

The solution is to define by hand your fft grid using the nr1, nr2, nr3 
parameters (choose them larger than those chosen by the program but with 
different dividers so as to comply with the symetry of your system). 
Another solution usually proposed is to increase the ecut_rho, but I 
prefer the first one.

Hope this helps,


mayank gupta a écrit :
> Dear PWSCF User's,
> I am doing phonon calculation of a material having 40 atoms in the
> unit cell and BCC structure. The phonon frequncy which should be
> degenerate are not coming out exactly degenerate, like transverse
> acoustic mode along 100 direction having frequency   0.44579   and
> 0.54478 Thz while they should have equal frequency. what should be the
> error which I made kindly suggest me. thanks in Advance.
> Mayank gupta
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