Raman calculation for bulk- Si and C

[tomohisa_ogawa]

Dear VASP experts
I’m interested in calculation of Raman spectrum. I found this site https://github.com/raman-sc/VASP, and tested bulk-Si tutorial, there exist alittle discrepancy, maybe OK. I also tested it with the diamond structure after PHONON calculation.

However, what I could observed is not consistent with the well know peak position. The discrepancy between the calculated spectrum and the one in several papers is roughly 100 cm-1, a little bit large discrepancy. I’m wondering I mistook some settings, or the current accuracy is limited by the current calculation methods?

If someone who are familiar with VASP/Raman know something, please teach me advice, hint about the above. I posted the spectra and files I used.

(I also found https://github.com/skelton-group/Phonop ... ree/master, and will test it as well, this could be bettwer software?)

[andreas.singraber]

Hello!

Unfortunately, I am no expert on the topic of Raman spectra. However, I noticed one potentially problematic setting in your INCAR file which may be the cause for the discrepancies you are describing. In the INCAR file you set

Code: Select all

ENCUT = 300.0

which is even below the minimum specified in the POTCAR file:

Code: Select all

ENMAX  =  433.983; ENMIN  =  325.487 eV

A too low energy cutoff may lead to various problems, described in detail on our Wiki, e.g. here and here. The pretty high external pressure reported in the OUTCAR file further hints at a bad convergence with respect to the energy cutoff. Before you try anything else I would suggest to perform a convergence test for ENCUT or at least try a single, much higher, value, e.g. ENCUT = 450.0.

Hope this is a helpful hint!

All the best,
Andreas Singraber

[tomohisa_ogawa]

Dear andreas.singraber
Thank you for your kind comments. I did not pay attention properly to such important parameter.

I modified ENCUT, set it to a little bit larger value than ENMAX, and I got the following.

Sharp peak in Raman with the diamnod exist in 1300, the difference with the ones from paperes is roughly 30cm-1, 30cm-1 is the similar discrepancy with the Si case I obsered. mybe this is usuall calculation performance of VASP?

As for the amorphous carbon (a-C) is relatively difference compared with, for aintance, the ones from this papers: https://www.researchgate.net/publicatio ... ous_Carbon , https://www.researchgate.net/publicatio ... rbon_Films , whereI can see large broad spectrum with the peak at around 1600. But, my calculated result does not look like.

Are there any known technique for the amorphous model? or this is just the reason that the structure is just difference from the ones in the paper as long as VASP calculates Raman of the cystal models properly? If you give me further comments, I would really appreciate it.
best regards

[alex]

Hello tomohisa_ogawa,

if you are in doubt about the quality of your chosen approximation for diamond or graphite, may I suggest a cheaper appraoch:
just calculate the phonon part (just the vibrational modes, no intensities) and see, where it goes if <ou change your setting here and there. Calculating Raman intensities does not help you here, because the modes' position does not change any more.

Hth,

alex

[tomohisa_ogawa]

Dear Alex
Thank you for the further comments, I will check your suggestion.
Best regards