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
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warfare.
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