<div dir="ltr">Hi Ankit Jain,<div><br></div><div><div>Germanium</div><div> &inputph</div><div> tr2_ph=1.0d-18</div><div> ldisp=.true.</div><div> nq1=10</div><div> nq2=10</div><div> nq3=1</div><div> prefix='GEPH_dfpt'</div>
<div> outdir='/scratch'</div><div> fildyn='./dfpt_10x/ge.dyn'</div><div> /</div></div><div><br></div><div><br></div><div>this activate a dispersive phonon calculation. If you switch the ldisp flag to .false. , if you unset nq1, nq2 and nq3 (you just neglect those) and if you specify q1 q2 and q3 at the very end of the input file (after the final slash /) you can calculate the frequencies at that q point. Ex.</div>
<div><br></div><div><div>Germanium</div><div> &inputph</div><div> tr2_ph=1.0d-18</div><div> prefix='GEPH_dfpt'</div><div> outdir='/scratch'</div><div> fildyn='./dfpt_10x/ge.dyn'</div><div> /</div>
</div><div>1.0 0.0 0.0</div><div><br></div><div>I suggest you to pick one q point where you found imaginary frequencies (from your frequencies output file) and just run ph.x < <a href="http://ph.in">ph.in</a> on it.</div>
<div><br></div><div>However I have to tell you that I am not familiar with phonon calculation on surfaces which I believe it's what you are doing right now. Am I correct? i see that your cell is stretched along "z" and you k-sampling is highly non-homogeneous. This may dramatically effects your phonon calculation. One last thing...did you relax through a vc-relax job or is that a pseudo-crystal experimental structure?</div>
<div><br></div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Sun, Aug 31, 2014 at 7:28 PM, ankit jain <span dir="ltr"><<a href="mailto:ankitjain.me.iitk@gmail.com" target="_blank">ankitjain.me.iitk@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div><div>Hello Vincenzo,<br><br></div>Thanks a lot for your reply.<div class=""><br><br> "First thing that i would try is to check if phonon calculations at a q point where matdyn gives you negative is correct or not"<br>
</div></div><div>How would I check this?<br><br></div><div>Also I am using tr2_ph = 1e-18 in the <a href="http://ph.in" target="_blank">ph.in</a> and conv_thr=1.0d-12 in the <a href="http://pw.in" target="_blank">pw.in</a> file.<br>
</div><div>My kgrid is 14*12*1 in the <a href="http://pw.in" target="_blank">pw.in</a> file.<br>
<br></div><div>Thanks,<br></div><div>Ankit Jain<br></div><div><br></div></div><div class="HOEnZb"><div class="h5"><div class="gmail_extra"><br><br><div class="gmail_quote">On Sun, Aug 31, 2014 at 1:13 PM, Vincenzo Verdolino <span dir="ltr"><<a href="mailto:vincenzo.verdolino@gmail.com" target="_blank">vincenzo.verdolino@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Dear,<br>
<br>
I have similar issues so i am not sure to be able to help you. First thing that i would try is to check if phonon calculations at a q point where matdyn gives you negative is correct or not. If yes that means that you k-samplin is good enough and it is only a matter of q mash for the FFT. Another check should be on the scf thrashold in <a href="http://ph.in" target="_blank">ph.in</a> input file<br>
<br>
<br>
Let me know<br>
<br>
Sent from my iPhone<br>
<div><div><br>
> On 31/ago/2014, at 17:03, ankit jain <<a href="mailto:ankitjain.me.iitk@gmail.com" target="_blank">ankitjain.me.iitk@gmail.com</a>> wrote:<br>
><br>
><br>
> Hello QE developers and users,<br>
><br>
> Recently, I am trying to calculate phonon dispersion of a 2D material using pw.x, ph.x, q2r.x and matdyn.x but I am getting very strange results.<br>
><br>
> In my calculations the phonon frequencies reported by ph.x are all real for all non-gamma phonon q-points. But, when I use matdyn.x to calculate phonon frequencies at other points (other than the one for which ph.x calculated frequencies), I get negative (imaginary) frequencies. And these negative frequencies are present for non-gamma points as can be seen in attached frequency file.<br>
><br>
> I am having difficulty in understanding these results and hoping someone could help.<br>
><br>
> Thank You,<br>
> Ankit Jain<br>
> PhD Candidate<br>
> IIT India<br>
><br>
</div></div>> <<a href="http://pw.in" target="_blank">pw.in</a>><br>
> <<a href="http://ph.in" target="_blank">ph.in</a>><br>
> <<a href="http://q2r.in" target="_blank">q2r.in</a>><br>
> <<a href="http://matdyn.in" target="_blank">matdyn.in</a>><br>
> <frequency><br>
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