Limitations of the Raman Calculation in ABINIT

Phonons, DFPT, electron-phonon, electric-field response, mechanical response…

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ckunka
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Joined: Thu Sep 01, 2016 10:21 pm

Limitations of the Raman Calculation in ABINIT

Post by ckunka » Thu Sep 29, 2016 11:52 pm

I seek to calculate the Raman spectra for a variety of boron-carbide structures. I have used plane waves, TM norm-conserving pseudopotentials, LDA XC (and checked against GGA XC), ABINIT DFPT (http://www.abinit.org/doc/helpfiles/for ... n_nlo.html), the harmonic approximation?, and a post-processing technique for Raman intensities (25 C, 532 nm).

My spectrum for the most popular crystal structure fits experimental data reasonably well, but a journal reviewer rejected my paper on the grounds that "Raman spectra calculated by "vanilla" DFT and harmonic approximation are rarely accurate."

Hence, here are a few questions:
1. What is the "best" way to calculate Raman?
2. What are the limitations of the ABINIT DFPT for calculating Raman? For example, are only norm-conserving pseudopotentials allowed? Can I use more than the gamma point? Should I use the transfer response functions instead of the non-linear optical properties?
3. How much impact on results and computational cost would the quasi-harmonic approximation have as compared to the harmonic approximation. If I understand correctly, the QHA allows temperature to shift frequencies slightly due to thermal expansion. I see ABINIT has the capability to do the QHA but do not see a tutorial for QHA for Raman.

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