<table cellspacing="0" cellpadding="0" border="0" ><tr><td valign="top" style="font: inherit;">Dear Ghosh,<br><br>Well, first of all, negative modes near q=2/3(111) means QE works correctly!!!<br>As bcc Zr is a high temperature phase, but DFT deals with T=0K, so, there is no direct way to get stable modes for bcc Zr (Ti, Hf). Instability near q=2/3(111) means the possiblity to transform to \omega phase. <br><br>At high temperatures phonon-phonon interaction (anharmonicity) becomes important, but it is quite difficult to implement it directly. In the paper you mentioned they do account anharmonic terms (see the papar). <br><br>What you can do? Of course, you can perform MD calculations at high temerature and even get Phonon DOS, but in this way you can not calculate phonon dispersions. <br>Not sure, D3 code can do this (phonon dispersions).<br>The only code that deals with phonons at high temperature is so called SCAILD code. But it is
not linear response method, it is based on direct (supercell) method. <br>I am thinking about is it possible to implement it in QE (sure, it will take time). <br>For reference see PRL 100, 095901 (2008).<br><br>Bests,<br>Eyvaz.<br><br>-------------------------------------------------------------------<br>
Prof. Eyvaz Isaev, <br>
<br>
DeTheoretical Physics Department, Moscow State Institute of Steel & Alloys, Russia,partment of Physics, Chemistry, and Biology (IFM), Linkoping University, Sweden <br>
isaev@ifm.liu.se, eyvaz_isaev@yahoo.com<br><br>--- On <b>Fri, 5/21/10, partha sarathi ghosh <i><parthasarathi13@gmail.com></i></b> wrote:<br><blockquote style="border-left: 2px solid rgb(16, 16, 255); margin-left: 5px; padding-left: 5px;"><br>From: partha sarathi ghosh <parthasarathi13@gmail.com><br>Subject: [Pw_forum] negative frequency<br>To: pw_forum@pwscf.org<br>Date: Friday, May 21, 2010, 1:41 PM<br><br><div id="yiv1406652866">Dear all,<br> Thank you xirainbow and Michael J. Mehlfor for your reply & suggestion .<br> I was trying to calculate longitudinal phonon dispersion of Zr along [111] direction. <br>I wanted to validate my calculation with experimental results for bcc-Zr (not hcp) given in the paper :Phy.Rev.B,vol-29,page 1575-1587(1984). <br>
But I am getting negative frequencies near q=2/3(1 1 1) and the over all appearance of the curve is different from the experimental one. I optimized the system w.r.t ecutwf, ecutrho , k-point sampling , volume ,degauss, conv_thr. I have also tried with different q-point sampling in <a rel="nofollow" target="_blank" href="http://ph.in">ph.in</a> file (like 4 4 4, 8 8 8) with tr2_ph=1.0d-12.<br>
My questions are :<br>1. Is DFTP suitable for studying particular phonon mode like in Zr ??<br>2. If yes then how to make my calculations efficient ?? <br>3. If not then any suggestion of other methods ??<br> Any one can please help me.<br>
Thanks in advance.<br>P. S. Ghosh<br>B.A.R.C.<br>INDIA <br>
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