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    <p>Sorry forgot half of the answer <br>
    </p>
    <p><br>
    </p>
    <p>acustic branches should be flat along k_z direction, I would
      check that is so. If it is not so, maybe you are adding non
      physical interactions between periodic images and this can cause
      the numerical instabilities.   I don't know whether your program
      is able to clean spurious interactions due to fake periodicity. <br>
       I hope this helps - pietro<br>
    </p>
    <div class="moz-cite-prefix">On 05/12/2016 09:52, Pietro Delugas
      wrote:<br>
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      <p>Then one of the 3 acustic branches ( probably the one you gave
        problems with ) should be flat. <br>
      </p>
      <p>I don't know how this phononpy works,  maybe there is a way to
        deal directly with 2D systems, otherwise just find a way to set
        to 0 all the interatomic force constants that  correspond to
        interactions between atoms staying in different periodic images
        of the slab.</p>
      <p><br>
      </p>
      <p>pietro   <br>
      </p>
      <br>
      <br>
      <div class="moz-cite-prefix">On 05/12/2016 09:33, Mohammed
        Ghadiyali wrote:<br>
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cite="mid:BN6PR06MB3042981603DD5690DB6E983ECD830@BN6PR06MB3042.namprd06.prod.outlook.com"
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          <p>Hi,</p>
          <p><br>
          </p>
          <p>The structure is two dimensional similar to bi-layer
            graphene. </p>
          <p><br>
          </p>
          <p>Regards,</p>
          <p><br>
          </p>
          <p>Ghadiyali Mohammed Kader</p>
          <p><br>
          </p>
          <p>Research Scholar</p>
          <p><br>
          </p>
          <p>University of Mumbai.</p>
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