<html><body><div style="font-family: arial, helvetica, sans-serif; font-size: 12pt; color: #000000"><div>Dear users and experts: </div><div><br data-mce-bogus="1"></div><div>I carried on a phonon calculation for a 20 atom strucuture using QE ph.x. However, in the ph.x output file, it show some messages as follow:</div><div><div> "Dynamical matrices for ( 2, 2, 2) uniform grid of q-points</div><div> ( 6 q-points):</div><div> N xq(1) xq(2) xq(3) </div><div> 1 0.000000000 0.000000000 0.000000000</div><div> 2 0.000000000 0.000000000 -0.397359503</div><div> 3 -1.222792984 -0.255394699 0.000000000</div><div> 4 -1.222792984 -0.255394699 -0.397359503</div><div> 5 -2.445585969 0.000000000 0.000000000</div><div> 6 -2.445585969 0.000000000 -0.397359503</div><div>Eigenvectors exchange needed 1 31</div><div>Eigenvectors exchange needed 3 33</div><div>Eigenvectors exchange needed 5 35</div><div>Eigenvectors exchange needed 7 37</div><div>Eigenvectors exchange needed 9 39</div><div>Eigenvectors exchange needed 11 41</div><div>Eigenvectors exchange needed 13 43</div><div>Eigenvectors exchange needed 15 45</div><div>Eigenvectors exchange needed 17 47 </div><div>"</div><div>I have two questions: </div><div>1.) I don't know that what exactly " Eigenvectors exchange needed " mean? </div><div>2) Will it cause a negative frequencies？ </div></div><div>Thank you.</div><div><br data-mce-bogus="1"></div><div>Regards,</div><div data-marker="__SIG_PRE__"><div><span style="color:rgb( 0 , 0 , 0 );font-family:'georgia' , serif;font-size:10pt">Yang Gang</span><br><span style="color:rgb( 0 , 0 , 0 );font-family:'georgia' , serif;font-size:10pt">Institut des Nanotechnologies de Lyon (INL) - UMR5270</span><br><span style="color:rgb( 0 , 0 , 0 );font-family:'georgia' , serif;font-size:10pt">Ecole Centrale de Lyon</span><br><span style="color:rgb( 0 , 0 , 0 );font-family:'georgia' , serif;font-size:10pt">36, Avenue Guy de Collongue</span><br><span style="color:rgb( 0 , 0 , 0 );font-family:'georgia' , serif;font-size:10pt">69 134 Ecully Cedex</span><br><span style="color:rgb( 0 , 0 , 0 );font-family:'georgia' , serif;font-size:10pt">Tél.: +33 (0)7 64 83 01 70</span><br><br></div></div></div></body></html>