Hi QE users,<br>
<br>I am a new user of QE. So, wanted to run some test cases for Si. Energy calculations were ok, but phonons are not. I am having trouble to get good result of optical phonons for 2-atom Si. Convergence of energy is obtained with <br>
<br>ecutwfc 25Ry<br>k-mesh 6 6 6 0 0 0 <br>norm-conserving PP<br clear="all">conv_thr = 1.0d-12<br>tr2_ph=1.0d-20<br><br>Even though the energy difference was reduced w.r.t. increasing ecutwfc and k-mesh, I could not find good values for optical phonons ((I did not worry about ASR for this Gamma calculation)) w.r.t these quantities. Here's what I get with 6x6x6 k-mesh without any offset of k-mesh:<br>
<br>15Ry 519.587327 [cm-1]<br>16Ry 516.231236 [cm-1]<br>20Ry 508.216250 [cm-1]<br>30Ry 510.229073 [cm-1]<br>35Ry 510.044651 [cm-1]<br>50Ry 509.893556 [cm-1]<br>100Ry 511.548608 [cm-1]<br><br>So some oscillations are there. But I would say it is converged around 510. The experimental value is ~519. So the QE result is quite different.<br>
<br>I read the tutorials and archieve, but I did not find the answer of my problem. <br><br><a href="http://users.aims.ac.za/~paolo/tutorial_phon.htmat">http://users.aims.ac.za/~paolo/tutorial_phon.htmat</a> <br>obtained the optical phonons to be 516.197992 [cm-1] for a ecutwfc of 16Ry. But at this ecutwfc the energy is not converged for Si. If I find ecutwfc to get energy convergence it becomes above 20Ry. But at this ecutwfc optical phonons are not getting better. I would expect better results at higher mesh and cutoff. I checked my result adding epsil=.true. as well. But result didn't change much.<br>
<br>Could you suggest what might improve the result.<br><br>Thanks in advance,<br>-- <br>Sophia<br>Univ. of California - Berkeley, USA<br>