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<p class="MsoNormal" dir="LTR" style="margin-left:18pt;text-align:left;direction:ltr"><span style="font-family:"Times New Roman","serif"">Dear Madam/Sir</span></p>

<p class="MsoNormal" dir="LTR" style="margin-left:18pt;text-align:left;direction:ltr"><span style="font-family:"Times New Roman","serif"">I have done some phonon calculations on carbon and silicon using
espresso-5.0.2.</span></p>

<p class="MsoNormal" dir="LTR" style="margin-left:18pt;text-align:left;direction:ltr"><span style="font-family:"Times New Roman","serif"">I have done harmonic and anharmonic calculations without any
difficulties.</span></p>

<p class="MsoNormal" dir="LTR" style="margin-left:18pt;text-align:left;direction:ltr"><span style="font-family:"Times New Roman","serif"">But when I apply various electric fields, phonon frequencies starts
from negative energies.</span></p>

<p class="MsoNormal" dir="LTR" style="margin-left:18pt;text-align:left;direction:ltr"><span style="font-family:"Times New Roman","serif"">For instance, phonon frequencies start from -150 meV for Si, while we
know that the material has no instability under electric field, because it’s
the main element in transistors.</span></p>

<p class="MsoNormal" dir="LTR" style="margin-left:18pt;text-align:left;direction:ltr"><span style="font-family:"Times New Roman","serif"">Therefore if you wouldn`t mind, I would like to ask am I allowed to
shift the diagrams to zero and make it align to the case that no electric field
was applied? </span></p>

<p class="MsoNormal" dir="LTR" style="margin-left:18pt;text-align:left;direction:ltr"><span style="font-family:"Times New Roman","serif"">It would be really nice of you to help me overcome this problem. </span></p>

<p class="MsoNormal" dir="LTR" style="margin-left:18pt;text-align:left;direction:ltr"><span style="font-family:"Times New Roman","serif"">Thanks for time and attentions</span></p>

<p class="MsoNormal" dir="LTR" style="margin-left:18pt;text-align:left;direction:ltr"><span style="font-family:"Times New Roman","serif"">Best regards </span></p>

<p class="MsoNormal" dir="LTR" style="margin-left:18pt;text-align:left;direction:ltr"><span style="font-family:"Times New Roman","serif"">Zohreh dabiri</span></p>

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