[Pw_forum] Can I obtain IR spectrum from CPMD trajectory in Quantum Espresso?

[Paolo Giannozzi]

>From the FFTs of the atomic velocity autocorrelation function you can
extract phonon frequencies, but you do not get "true" spectra

Paolo

On Wed, Jul 29, 2015 at 6:08 PM, Pascal Boulet <pascal.boulet at univ-amu.fr>
wrote:

> Hello,
>
> Isn’t it possible to get IR spectra from the FFTs of the atomic velocity
> autocorrelation function?
>
> Pascal
>
> Le 29 juil. 2015 à 17:58, Nicola Marzari <nicola.marzari at epfl.ch> a écrit
> :
>
>
> Dear Yin,
>
>
> you need to calculate at every time step the total dipole in the unit
> cell, and then do the fourier transform of the autocorrelation - see e.g.
> here: http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.81.1235 or many
> other papers published on the topic.
>
> Note that if you do Car-Parrinello MD, rather than Born-Oppenhimer MD,
> the vibrational frequencies can be slightly softened due to the fact
> that the ions are dressed (and slowed down) by the coefficients of the
> plane waves. The smaller the fictitious mass, the small the effect.
>
> Now, I'm not sure how to write the polarization at every time step -
> maybe others can comment or add to the documentation. For PWSCF
> you'd switch on lberry, but I'm not sure if you can do it during an
> md (if now, maybe switch on lelfield, and then keep the applied field
> equal to zero).
>
> In CP I suppose you could switch tefield on, but still have zero
> applied field.
>
> Any other suggestion? Also, I think that tefield in CP and in PWSCF
> mean different things - shall we switch the name in CP to lelfield?
>
>   nicola
>
>
>
> On 29/07/2015 15:56, liyincumt at gmail.com wrote:
>
> Dear Quantum Espresso developers,
>
> I want to obtain IR spectra of crystals at room temperature. Considering
> the temperature effect, I decided to use CPMD.  I wonder whether it is
> possible to get IR spectra from CPMD trajectory in Quantum Espresso?
> What's more, can I involve vdw-correction in CPMD? If yes, can IR
> spectra be calculated in the presence of vdw-correction?
>
> Thank you very much!
>
> Best Regards,
> ------------------------------------------------------------------------
> Dr. Yin Li
> Department of Biophysics,Medical School, University of Pecs,
> No.12 Szigeti Street, Pecs, H-7624, HUNGARY
> Phone: +36-72-535271/36271
>
>
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>
> --
> ----------------------------------------------------------------------
> Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
> Director, National Centre for Competence in Research NCCR MARVEL, EPFL
> http://theossrv1.epfl.ch/Main/Contact http://nccr-marvel.ch/en/project
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>
> --
> Pascal Boulet *- MCF HDR, Resp. L1 MPCI - DEPARTEMENT CHIMIE*
> Aix-Marseille Université - ST JEROME - Avenue Escadrille Normandie Niemen
> - 13013 Marseille
> Tél: +33(0)4 13 55 18 10 - Fax : +33(0)4 13 55 18 50
> Site : http://allos.up.univ-mrs.fr/pascal - Email :
> pascal.boulet at univ-amu.fr
> *Afin de respecter l'environnement, merci de n'imprimer cet email que si
> nécessaire.*
>
>
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-- 
Paolo Giannozzi, Dept. Chemistry&Physics&Environment,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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