<div dir="ltr"><div>From the FFTs of the atomic velocity autocorrelation function you can extract phonon frequencies, but you do not get "true" spectra<br><br></div>Paolo<br></div><div class="gmail_extra"><br><div class="gmail_quote">On Wed, Jul 29, 2015 at 6:08 PM, Pascal Boulet <span dir="ltr"><<a href="mailto:pascal.boulet@univ-amu.fr" target="_blank">pascal.boulet@univ-amu.fr</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word">Hello,<div><br></div><div>Isn’t it possible to get IR spectra from the FFTs of the atomic velocity autocorrelation function?</div><div><br></div><div>Pascal</div><div><br><div><div>Le 29 juil. 2015 à 17:58, Nicola Marzari <<a href="mailto:nicola.marzari@epfl.ch" target="_blank">nicola.marzari@epfl.ch</a>> a écrit :</div><br><blockquote type="cite"><br>Dear Yin,<br><br><br>you need to calculate at every time step the total dipole in the unit<br>cell, and then do the fourier transform of the autocorrelation - see e.g.<br>here: <a href="http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.81.1235" target="_blank">http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.81.1235</a> or many<br>other papers published on the topic.<br><br>Note that if you do Car-Parrinello MD, rather than Born-Oppenhimer MD,<br>the vibrational frequencies can be slightly softened due to the fact<br>that the ions are dressed (and slowed down) by the coefficients of the<br>plane waves. The smaller the fictitious mass, the small the effect.<br><br>Now, I'm not sure how to write the polarization at every time step -<br>maybe others can comment or add to the documentation. For PWSCF<br>you'd switch on lberry, but I'm not sure if you can do it during an<br>md (if now, maybe switch on lelfield, and then keep the applied field<br>equal to zero).<br><br>In CP I suppose you could switch tefield on, but still have zero<br>applied field.<br><br>Any other suggestion? Also, I think that tefield in CP and in PWSCF<br>mean different things - shall we switch the name in CP to lelfield?<br><br><span style="white-space:pre-wrap"> </span><span style="white-space:pre-wrap"> </span><span style="white-space:pre-wrap"> </span><span style="white-space:pre-wrap"> </span>nicola<br><br><br><br>On 29/07/2015 15:56, <a href="mailto:liyincumt@gmail.com" target="_blank">liyincumt@gmail.com</a> wrote:<br><blockquote type="cite">Dear Quantum Espresso developers,<br><br>I want to obtain IR spectra of crystals at room temperature. Considering<br>the temperature effect, I decided to use CPMD. I wonder whether it is<br>possible to get IR spectra from CPMD trajectory in Quantum Espresso?<br>What's more, can I involve vdw-correction in CPMD? If yes, can IR<br>spectra be calculated in the presence of vdw-correction?<br><br>Thank you very much!<br><br>Best Regards,<br>------------------------------------------------------------------------<br>Dr. Yin Li<br>Department of Biophysics,Medical School, University of Pecs,<br>No.12 Szigeti Street, Pecs, H-7624, HUNGARY<br>Phone: <a href="tel:%2B36-72-535271" value="+3672535271" target="_blank">+36-72-535271</a>/36271<br><br><br>_______________________________________________<br>Pw_forum mailing list<br><a href="mailto:Pw_forum@pwscf.org" target="_blank">Pw_forum@pwscf.org</a><br><a href="http://pwscf.org/mailman/listinfo/pw_forum" target="_blank">http://pwscf.org/mailman/listinfo/pw_forum</a><br><br></blockquote><br>-- <br>----------------------------------------------------------------------<br>Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL<br>Director, National Centre for Competence in Research NCCR MARVEL, EPFL<br><a href="http://theossrv1.epfl.ch/Main/Contact" target="_blank">http://theossrv1.epfl.ch/Main/Contact</a> <a href="http://nccr-marvel.ch/en/project" target="_blank">http://nccr-marvel.ch/en/project</a><br>_______________________________________________<br>Pw_forum mailing list<br><a href="mailto:Pw_forum@pwscf.org" target="_blank">Pw_forum@pwscf.org</a><br><a href="http://pwscf.org/mailman/listinfo/pw_forum" target="_blank">http://pwscf.org/mailman/listinfo/pw_forum</a><br></blockquote></div><br><div>
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