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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>
    </div>
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cite="mid:CAE_UR+UnCNFCUtB6PNdGUkr=ikhcRh6T0LAe3u+1vmtpmf4=2g@mail.gmail.com">
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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>
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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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