[Pw_forum] negative frequency in high q-mesh

[Wei-Bing Zhang]

Dear all

Recently, I am trying to performed a DFPT calculation to get the
phonon of silicene using PWSCF. Actually, i can obtain reasonable
phonon
dispersion of silicene using q-points up to 6x6x1 (including 4x4x1)
but a negative frequency always appear in the phonon calculations with
q-points of 8x8x1, 9x9x1, 12x12x1. I have tested  a lot of parameter
in the calculations including the k-points in scf calculations with
(24x24x1) and also (32x32x1), conv_thr (1d-10,1d-12) and also tr2_ph(
1.0d-18,1.0d-20,1.0d-22). And I also tested the different potential
(including Si.pbe-n-van.UPF, Si.pbe-rrkj.UPF, Si.pz-vbc.UPF) and the
scheme using in relaxed the structure (i.e vc-relax directly and
atomic relaxation with different lattice parameters). Unfortunately, I
can only get the reasonable phonon dispersion with q-mesh below 6x6x1.
In my opinion, the phonon should be converged with q-mesh. More q-mesh
one used more reliable phonon one can obtain. The main test input and
output files are enclosed. Could you help me check them and tell me
what i missed in the calculation?  Any suggestion will be appreciated!


Thanks and best wishes.


Wei-Bing Zhang


--
----------------------------------------------------------------------
Dr. Wei-Bing Zhang

School of Physics and Electronic Sciences
Changsha University of Science and Technology,
Changsha,  People’s Republic of China
Phone : +86 186 73108538
E-mail: zwb256 at hotmail.com, weibingzhangavh at gmail.com
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