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<p>Hello,</p>
<p>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.<br>
</p>
<p>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:</p>
<p>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.</p>
<p>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.<br>
</p>
<p>3. Metallic systems and resonant Raman is a completely different
story.<br>
</p>
<p>kind regards<br>
</p>
<div class="moz-cite-prefix">On 10/07/2025 01:03, Gulshan Kumar via
users wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CABjxb4SAtb52BnSNK+3_RhE5ORBUhUYzfkheWv1nxRqs2jFsaQ@mail.gmail.com">
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<div>Hi everyone,<br>
How can we do post-processing for second-order Raman modes? If
anyone has done it, please provide me with details.<br>
I have done ph.x, q2r.x, and matdyn calculations. </div>
<div><br>
Thank you<br>
gulshan</div>
</div>
<br>
<fieldset class="moz-mime-attachment-header"></fieldset>
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IR IMPMC - CNRS UMR 7590 / Sorbonne Université / MNHN <br>
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