<div dir="ltr"><div>Dear Lorenzo,</div>I apologize for getting back so late on this. I had posted a reply but looks like the email bounced back due to some size restriction on the attachment. But to answer your question, I did not find a negative frequency anywhere else in the q-point grid. I am attaching my scf, ph, q2r and matdyn input files. I am facing the same problem with another crystal structure of SiO2 as well (cristobalite). <span style="font-size:13px">I feel like I am making some mistake in the input files that is probably important for polar materials.</span><div><br></div><div><span style="font-size:13px">I would really appreciate it if someone could look into it. Thanks!</span><br></div><div><span style="font-size:13px">Sridhar</span></div><div><span style="font-size:13px">Purdue University</span></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Mon, Nov 17, 2014 at 2:34 AM, Lorenzo Paulatto <span dir="ltr"><<a href="mailto:lorenzo.paulatto@impmc.upmc.fr" target="_blank">lorenzo.paulatto@impmc.upmc.fr</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
Dear Sridhar,<br>
normally the sum rules imposed by matdyn are enough to set the
acoustic bands at gamma to exactly zero. If this is not the case, it
normally means that you have some negative frequency somewhere else
in the q-point grid. Please check the other matdyn files to see if
this is the case: the frequencies are at the end of the file.<br>
<br>
kind regards<div><div class="h5"><br>
<br>
<br>
<div>On 17/11/14 02:03, Sridhar Sadasivam
wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">Dear Stefano,
<div><br>
</div>
<div>Thanks for your reply. Here are the frequencies at gamma
point when I set zasr and asr to 'simple'</div>
<div>
<div> -12.8981 -12.8981 -4.5998 142.6073 168.0267
168.0267</div>
<div> 249.0588 249.0588 340.4760 344.4668 379.2589
379.2589</div>
<div> 417.0274 417.0274 429.6273 457.2113 657.8050
657.8050</div>
<div> 746.0641 755.8553 755.8553 1032.5200 1032.5200
1037.0491</div>
<div> 1048.5362 1136.7760 1136.7760</div>
</div>
<div><br>
</div>
<div>Thanks,</div>
<div>Sridhar</div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Sun, Nov 16, 2014 at 5:25 AM,
stefano de gironcoli <span dir="ltr"><<a href="mailto:degironc@sissa.it" target="_blank">degironc@sissa.it</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000">
<div>dear Sridhar Sadasivam<br>
can you try to set zasr and asr to 'simple' in the
q2r+matdyn case and report the results ?<br>
thanks<br>
stefano
<div>
<div><br>
<br>
<br>
On 11/15/2014 11:49 PM, Sridhar Sadasivam wrote:<br>
</div>
</div>
</div>
<blockquote type="cite">
<div>
<div>
<div dir="ltr">Dear espresso users and developers,
<div><br>
</div>
<div>I am trying to obtain the phonon dispersion
of alpha-quartz (SiO2) which has a 9 atom unit
cell. I ran ph.x on a 3 x 3 x 3 q-point grid. I
am seeing some strange behavior where there
seems to be a difference in the frequencies
output from dynmat.x and matdyn.x. I will
explain my problem below. </div>
<div><br>
</div>
<div>I diagonalized the dynamical matrix at the
gamma point using dynmat.x with acoustic sum
rule set to crsytal. Since SiO2 is a polar
material, LO-TO splitting will need to be
considered at the gamma point but I am ignoring
it currently (I did not specify any direction
for LO-TO splitting in the dynmat.x input). This
gave me the following 27 frequencies (in cm^-1)
at the gamma point: (0,0,0, 152.24, 175.57,
175.57, 256.32, 256.32, 341.09, 354.41, 376.86,
376.86, 419.01, 419.01, 435.37, 455.70, 659.12,
659.12, 744.54, 754.62, 1034.38, 1034.38,
1040.20, 1051.12, 1139.60, 1139.60) </div>
<div><br>
</div>
<div>I also used q2r.x (with zasr = 'crystal') to
obtain the force constants and then used
matdyn.x (with asr = 'crystal') to obtain the
frequencies at the gamma point. However with
matdyn.x, I obtain the following frequencies at
the gamma point:
(-29.4088,-22.2535,-22.2535,152.2412,175.8141,175.8141,
256.3373, 256.3373, 341.0892, 355.4227, 376.0495
, 376.0495, 418.6344, 418.6344, 435.3710,
453.6862, 659.7607, 659.7607, 746.4843,
754.8867, 754.8867, 1034.1751, 1034.1751,
1039.7590, 1051.1207, 1139.8191, 1139.8191)</div>
<div><br>
</div>
<div>Could someone please explain why I am getting
imaginary frequencies from matdyn.x while
dynmat.x gives real frequencies at the same
q-point (gamma in this case)? </div>
<div><br>
</div>
<div>Thanks,<br>
Sridhar</div>
<div>Purdue University</div>
<div><br>
</div>
</div>
<br>
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</div></div><span class="HOEnZb"><font color="#888888"><pre cols="72">--
Dr. Lorenzo Paulatto
IdR @ IMPMC -- CNRS & Université Paris 6
+33 (0)1 44 275 084 / skype: paulatz
<a href="http://www-int.impmc.upmc.fr/~paulatto/" target="_blank">http://www-int.impmc.upmc.fr/~paulatto/</a>
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