<html><body><div style="color:#000; background-color:#fff; font-family:HelveticaNeue-Light, Helvetica Neue Light, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif;font-size:16px"><div dir="ltr" id="yui_3_16_0_1_1419680440732_2991"><span id="yui_3_16_0_1_1419680440732_3072">Dear all,</span></div><div id="yui_3_16_0_1_1419680440732_3073" dir="ltr"><br><span id="yui_3_16_0_1_1419680440732_3072"></span></div><div id="yui_3_16_0_1_1419680440732_3074" dir="ltr"><span id="yui_3_16_0_1_1419680440732_3072">I am trying to calculate phonon dispersion. Firstly, I try to run phonon calculation at gamma point and use dynmat.x to get the vibration frequency at gamma. It imposed well frequencies with zero for 3 acoustic modes. <br></span></div><div id="yui_3_16_0_1_1419680440732_3075" dir="ltr"><span id="yui_3_16_0_1_1419680440732_3072"><br></span></div><div id="yui_3_16_0_1_1419680440732_3076" dir="ltr"><span id="yui_3_16_0_1_1419680440732_3072">After that, I try to calculation phonon using grid and calculate force by q2r.x. The phonon dispersion was calculated by matdyn.x. However, I got a negative value of acoustic modes at gamma point. The value is around -20cm-1. <br></span></div><div id="yui_3_16_0_1_1419680440732_3824" dir="ltr"><br><span id="yui_3_16_0_1_1419680440732_3072"></span></div><div id="yui_3_16_0_1_1419680440732_3417" dir="ltr"><span id="yui_3_16_0_1_1419680440732_3072">I have used asr=simple for all calculations. <br></span></div><div id="yui_3_16_0_1_1419680440732_4232" dir="ltr"><br><span id="yui_3_16_0_1_1419680440732_3072"></span></div><div id="yui_3_16_0_1_1419680440732_4234" dir="ltr"><span id="yui_3_16_0_1_1419680440732_3072">Could you please give me some hints to overcome this issue? It seems negative value originated from numerical error (dynmat.x imposed well zero frequency) but I can not understand why they imposed negative values by matdyn.x. <br></span></div><div id="yui_3_16_0_1_1419680440732_4642" dir="ltr"><br><span id="yui_3_16_0_1_1419680440732_3072"></span></div><div id="yui_3_16_0_1_1419680440732_4643" dir="ltr"><span id="yui_3_16_0_1_1419680440732_3072">I would like to thank for all your instructions. </span></div><div id="yui_3_16_0_1_1419680440732_3077"> </div><div id="yui_3_16_0_1_1419680440732_3079"><div id="yui_3_16_0_1_1419680440732_3078">Best regards,
<br>-----------------------------------------------
<br>Dinh Loc Duong , Ph.D
<br>Postdoctoral Researcher, Department of Nanoscience
<br>Max Planck Institute for Solid State Research
<br>Heisenbergstrasse 1, D-70569 Stuttgart, Germany
<br>Email: mambom1902@yahoo.com, l.duong@fkf.mpg.de
<br></div></div></div></body></html>