[QE-users] Raman spectra for metallic system in QE

[Lorenzo Paulatto]

> More specifically, is it true that calculating Raman spectra for metals is not implemented in Quantum Espresso.

Hello,

the calculation of both dielectric matrix and Raman tensor in phonon is 
only available for insulators and semiconductors, it also lacks gradient 
correction and spin polarization. If you try to fool the code by not 
using a smearing, you will get nonsensical dielectric tensor, as you 
did. You can compute the frequency-dependent epsilon with the code 
epsilon.x, this will allow you to compute the Raman response for a given 
laser frequency.

You can compute the Raman tensor of metals using finite differentiation 
of the frequency-dependent dielectric tensor along phonon displacement. 
This can easily be done by hand:
1. take the displacements "v" of the phonon from the dynamical matrix 
file (they are 3*n complex numbers, but at Gamma the imaginary parts are 
zero)

2. move the atoms up and down this direction by a small amount (i.e. 
±1.d-3 bohr)

3. compute the dielectric tensor in both cases (using epsilon.x)

4. compute the derivative. (epsilon(r+h v)-epsilon(r-h v))/2h

This will give you the Raman tensor in cartesian axes for the given phonon.


A more efficient alternative, is via the finite differentiation of 
forces w.r.t an external electric field, as explained in 
doi:10.1038/s41524-024-01236-3. The methodology should be available via 
the Aiida interface.

kind regards


> How does that work in VASP (for metals)?

No idea, you should its developers.


-- 
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/
23-24/423 B115, 4 place Jussieu 75252 Paris CX 05
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