<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Dear Sophia & Alexandra: <div><br></div><div>I am not sure that group theory (which I am fond of) would help much here. For a disordered system modeled by a supercell, I see few other choices than estimating the Raman activity of each mode, and then sum the contributions from all the modes. The method by Mauri and coworkers (whose implementation status in QE is unknown to me) seems particularly suitable in this case. For those who do not know, the (non-resonant) Raman activity is given by the variation of the polarizability linearly induced by each vibrational mode (here is where selection rules ultimately come from). This "traditional" way of calculating the Raman activity is not very practical for large supercells, because it requires several polarizability calculations for each phonon mode. An alternative would be 3rd order perturbation theory, which used to be implemented in QE. A bit awkward because of complications arising from the infamous q->0 limits that plague any polarizability calculations in infinite systems. The third (and most elegant) way is Mauri and collaborators'. The first derivative of the polarizability with respect to an atomic displacements is a third derivative of the energy, once with respect to a nuclear position, twice with respect to the strength of the electroc field. By changing the order of differentiation, this can be seen as the second dericative of the force acting on atoms with respect to an applied alectric field. This can be computed by 2nd order perturbation theory in the strength of the field.<div><br></div><div>An alternative is to rely on some polarizable-bond model for the polarizability and to estimate its derivative with respect to nuclear distortions using such a semi-empirical model.</div><div><br></div><div>Hope this helps.</div><div>Stefano</div><div><br><div><div>On Jun 23, 2009, at 10:31 AM, <a href="mailto:alexandra.carvalho@epfl.ch">alexandra.carvalho@epfl.ch</a> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div>Dear Sophia,<br><br>To select which modes are Raman active you must use the selection rules.<br>The following paper may be of help (see also the tables in it):<br><br>Rosseau et al. Journal of Raman Spectroscopy 10, 253 (1981)<br><br>If you are starting, I recommend having a look at the selection rules<br>for molecules, for example in Tinkham, Group theory and Quantum <br>mechanics (Dover, 1992,NY) pp248.<br><br>Most likelly, you will be interested in the allowed transitions with highest<br>frequency.<br><br>Hope this helps,<br><br>Alexandra<br><br><br>Quoting Jonathan <<a href="mailto:sophia.nishad@gmail.com">sophia.nishad@gmail.com</a>>:<br><br><blockquote type="cite">Dear users,<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">I computed phonons only at the gamma point for a 64-atom binary <br></blockquote><blockquote type="cite">alloy. How do I separate the three main Raman modes.<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">Would it be possible to get some help?<br></blockquote><blockquote type="cite">thanks,<br></blockquote><blockquote type="cite">--<br></blockquote><blockquote type="cite">Sophia<br></blockquote><blockquote type="cite">UCB, USA<br></blockquote><blockquote type="cite"><br></blockquote><br><br><br>-- <br><br>Alexandra Carvalho,<br>Ceramics Laboratory<br>Swiss Federal Institute of Technology of Lausanne (EPFL)<br><br>_______________________________________________<br>Pw_forum mailing list<br><a href="mailto:Pw_forum@pwscf.org">Pw_forum@pwscf.org</a><br>http://www.democritos.it/mailman/listinfo/pw_forum<br></div></blockquote></div><br><div> <span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; 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"><div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; ">---</span></font></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; ">Stefano Baroni - SISSA</span></font><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; "> </span></font><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; ">&</span></font><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; "> </span></font><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; ">DEMOCRITOS National Simulation Center - Trieste</span></font></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; 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">La morale est une logique de l'action comme la logique est une morale de la pensée - Jean Piaget</span></font></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; "><br></span></font></div></span></div></span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; ">Please, if possible, don't</span></font><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; "> </span></font><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; ">send me MS Word or PowerPoint attachments</span></font></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; ">Why? See:</span></font><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; "> </span></font><font class="Apple-style-span" color="#0023E9" size="3"><span class="Apple-style-span" style="font-size: 12px; "><a href="http://www.gnu.org/philosophy/no-word-attachments.html">http://www.gnu.org/philosophy/no-word-attachments.html</a></span></font></div><div><font class="Apple-style-span" color="#0023E9" size="3"><span class="Apple-style-span" style="font-size: 12px; "><br class="webkit-block-placeholder"></span></font></div></div></div></span><br class="Apple-interchange-newline"></div></span><br class="Apple-interchange-newline"> </div><br></div></div></body></html>