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<p>Hi,</p>
<p>I believe that to claim if a structure is stable or not it is
unsafe to rely on the Fourier interpolation (q2r and PlotPhon
code), specially when they depend so much on the application of
the sum rule. Instead, you should run a calculation on the
specific q point that is unstable. This calculation, unless at
Gamma where ASR is trivial on the acoustic modes, is unaffected by
ASR. <br>
</p>
<p>If you find differences between ASR in the interpolation, try
increasing:<br>
</p>
<p>1) The parameters as tr2_ph (I usually use 1d-18 but it may
depend on the system) and conv_thr in the pw (I never use values
bigger than 1d-14 for phonons, sometimes even smaller values are
required)<br>
</p>
<p>2) the Q grid in which you ran the calculation is too coarse
grain, this creates problems in the Fourier transform operated by
q2r, try to use finer meshes.</p>
<p>One of the strenght of quantum-espresso PHonon package is that it
can compute the phonons at any q point without using a
commensurate supercell. <br>
</p>
<p>So, to indentify an instability you should see an imaginary mode
directly in the first output of the phonon package, not only in
the interpolated band structure (you can grep the 'freq' keyword
on the output dynamical matrix, remember that you will have 3
acoustic modes at gamma with random values, you can use them to
check the convergence of the parameters).<br>
</p>
<p>So if you find a q point at which you have an imaginary frequency
after the Fourier interpolation, and this q point was not present
in the original q mesh you gave, try running a ph.x with just that
q point.</p>
<p>Moreover, pay attention to the structure and cell relaxation
before running the phonons, in many cases I found systems in which
you must specify a forc_conv_thr much smaller than the default
value (e.g. 1d-5) to avoid fake imaginary modes appearing in the
calculation.</p>
<p>Bests,</p>
<p>Lorenzo<br>
</p>
<p><br>
</p>
<div class="moz-cite-prefix">Il 20/09/20 15:16, Suresh A ha scritto:<br>
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<div>Dear All,</div>
<div> I am asking this question to know whether I
have made a mistake in my calculations during my run using
phonon code in pwscf. I also went through previous posts but
could not understand as I am not good at phonon theory. <br>
</div>
<div>I have run calculations for three alloys. Two of them give
positive frequencies for zasr='crystal' as well as for
zasr='simple' but for one alloy it gives negative frequency
for zasr='simple' and positive frequency for zasr='crystal'.
Due to curiosity I just simply increased the convergence
threshold for phonon calculation alone (not in any other part
in scf) from tr2_ph = 1.0d-12, to tr2_ph = 1.0d-14, Now the
results are reversed. I got positive frequency for
zasr='crystal' and negative frequency for zasr='simple. I want
to ask two questions from this,</div>
<div><br>
</div>
<div>1. May I claim my alloy is a stable one, because the
previous report says that it is an unstable structure?</div>
<div>2.I want to know whether I have made any mistake in my run?
<br>
</div>
<div> Following is my procedure I used for my calculations:</div>
<div>1. SCF <br>
</div>
<div> 2. Phonon run<br>
</div>
<div>3. q2r with zasr = simple and zasr= crystal</div>
<div>4. PlotPhon code for postprocessing (I have run for other
two alloys for which there is no negative frequency for zasr=
simple and crystal.) Here I simply used this code immediately
after I ran q2r by simply changing asr='crystal' to simple
in the folder PlotPhon-> Scripts-> Plot_run. <br>
</div>
<div>Did i make any mistake here?<br>
</div>
<div>
Thanks and regards</div>
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Yours Sincerely,<br>
<br>
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<div
style="text-align:right"> A.Suresh,</div>
<div
style="text-align:right">Post-Doctoral
Fellow</div>
<div
style="text-align:right">Department
of Physics</div>
<div
style="text-align:right">National
Institute of
Technology</div>
<div
style="text-align:right">Tiruchirappalli</div>
<div
style="text-align:right">Tamil
Nadu,</div>
<div
style="text-align:right">India<br>
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