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<br><div><div>On Mar 21, 2014, at 8:29 AM, Paolo Giannozzi <<a href="mailto:paolo.giannozzi@uniud.it">paolo.giannozzi@uniud.it</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div style="font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;">On Tue, 2014-03-18 at 16:57 +0000, Kan-Ju Lin wrote:<br><br><blockquote type="cite">To facilitate the frequency calculation, I used nat_todo = 1 to only<br>allowing the helium to be displaced [...]<br></blockquote><br><blockquote type="cite">My questions are:<br>1. Why are there 3 additional frequencies? Shouldn’t there only be 3<br>(omega 73~75) due to the DoF of one atom?<br></blockquote><br>nat_todo means that the calculation is performed only for irreducible<br>representations containing a displacement of the chosen atoms(s).<span class="Apple-converted-space"> </span><br>In the output of the phonon code, you should see how many irreps are<br>used and their degeneracy. If there are 6 frequencies, there should<span class="Apple-converted-space"> </span><br>be a total of 6 irreps. What you get as frequencies may not have any<br>special physical meaning, with the exception of some special cases<br>(e.g. molecule on a surface with well-separated modes)<br><br>Paolo<br><br><blockquote type="cite">2. Why are the three additional frequencies negative? Shouldn’t there<br>be one negative frequency for the saddle point, and 2 positive<br>frequencies? and no negative frequency for the energy minimum?<br><br><br>Please let me know if I understand this incorrectly, or I am missing<br>some settings.<br><br><br><br><br>--<br>Kan-Ju Lin, Postdoctoral Associate<br>MIT Materials Science and Engineering<br>Office 4-053 / <a href="mailto:kjlin@mit.edu">kjlin@mit.edu</a><br><br><br><br><br><br><br>_______________________________________________<br>Pw_forum mailing list<br><a href="mailto:Pw_forum@pwscf.org">Pw_forum@pwscf.org</a><br><a href="http://pwscf.org/mailman/listinfo/pw_forum">http://pwscf.org/mailman/listinfo/pw_forum</a><br></blockquote><br>--<span class="Apple-converted-space"> </span><br>Paolo Giannozzi, Dept. Chemistry&Physics&Environment,<span class="Apple-converted-space"> </span><br>Univ. Udine, via delle Scienze 208, 33100 Udine, Italy<br>Phone +39-0432-558216, fax +39-0432-558222<span class="Apple-converted-space"> </span><br><br>_______________________________________________<br>Pw_forum mailing list<br><a href="mailto:Pw_forum@pwscf.org">Pw_forum@pwscf.org</a><br><a href="http://pwscf.org/mailman/listinfo/pw_forum">http://pwscf.org/mailman/listinfo/pw_forum</a></div></blockquote></div><br></div></body></html>