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

[Nicola Marzari]

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