[QE-users] Second order Raman modes

[Gulshan Kumar]

Thank you for your reply. It helped me a lot to understand it.

On Thu, Jul 10, 2025 at 12:38 PM Lorenzo Paulatto <lorenzo.paulatto at cnrs.fr>
wrote:

> Hello,
>
> you cannot do second order Raman with ph.x alone, because second order
> Raman come from two-phonon excitation, and ph.x does single phonon. If you
> do the phonon calculation at Gamma with lraman=.true. and then process the
> file with dynmat (or by hand, it is a really simple formula) you can get
> the Raman cross-section for each individual phonons.
>
> You can however compute two-phonon vinrational spectra using the d3q.x
> code and tools. The "spectral weight" functionality of the linewidth code
> (d3_lw.x), as shown the in the MgO example that should be available in the
> last version. There are three caveats:
>
> 1. The Raman cross-section is not taken into account, but it can easily be
> included afterwards : the code produce one specrum per phonon band, it is a
> reasonable approximation to just use the cross-section of the unperturbed
> phonon (as produced by dynmat.x) and multiply by hand.
>
> 2. This code can do two-phonon spectra, which you then couple to light via
> the Raman cross-section. I.e. the photon excite a phonon which the decays
> to two phonons. There is an additional term, which correspond to direct
> excitation of two phonons from one photon via a higher order dielectric
> response. The equation is written in R. A. Cowley, Reports on Progress in
> Physics 31, 123 (1968), and the matrix element can be computed by doing
> finite difference derivation of the Raman cross-section, I know it has been
> done in very simple materials, but afaik nobody as ever published a code to
> do it easily.
>
> 3. Metallic systems and resonant Raman is a completely different story.
>
> kind regards
> On 10/07/2025 01:03, Gulshan Kumar via users wrote:
>
> Hi everyone,
> How can we do post-processing for second-order Raman modes? If anyone has
> done it, please provide me with details.
> I have done ph.x, q2r.x, and matdyn calculations.
>
> Thank you
> gulshan
>
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> --
> Dr. Lorenzo Paulatto
> IR IMPMC - CNRS UMR 7590 / Sorbonne Université / MNHN
> phone: +33 (0)1 442 79822 / telegram: lpaulatto
> http://www.impmc.upmc.fr/~paulatto/ - https://anharmonic.github.io/
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>
> _______________________________________________________________________________
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> worldwide who are victims of terrorism, military aggression, and
> indiscriminate warfare.
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