[QE-users] Second order Raman modes

[Lorenzo Paulatto]

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
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