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Dear VASP team and community,

I am currently investigating a XANEs calculation for a relaxed quinone molecule. I attempted to follow the tutorial provided for diamond on the VASP wiki, however the instructions for testing different supercell sizes does not apply for my model which is not periodic. What I am not sure about is if I have specified the correct excited carbon atom for a quinone molecule. Below are my POSCAR and INCAR input files:

INCAR
System = QUINONE
ALGO = FAST
ISMEAR = 0; SIGMA = 0.05;
ICORELEVEL = 2
CLNT = 1
CLN = 1
CLL = 0
CLZ = 1.0
CH_LSPEC = .TRUE.
CH_SIGMA = 0.5
NBANDS = 1500
LREAL = A

POSCAR
1.00000000000000
20.0000000000000000 0.0000000000000000 0.0000000000000000
0.0000000000000000 20.0000000000000000 0.0000000000000000
0.0000000000000000 0.0000000000000000 20.0000000000000000
C C O H
1 5 2 4
Direct
0.3957583036356592 0.4828314031576154 0.5163651796613230
0.4303095230719756 0.4173651190066814 0.5128471485192949
0.4975080425005751 0.4131368365324600 0.5154301686704765
0.5395852502472201 0.4737748036596649 0.5220607120335120
0.5050462472013966 0.5392491350201754 0.5254159410670878
0.4378504314535588 0.5434747738213744 0.5228385194281890
0.6012575847734358 0.4698943662642115 0.5248066882121984
0.3340737370653957 0.4867185691664748 0.5139202939205481
0.3980934994848961 0.3735651481141350 0.5080990128920908
0.5242828014177766 0.3656354115673711 0.5129083116761589
0.5372660183860186 0.5830498124346483 0.5301165802745784
0.4110985477621018 0.5909846132551869 0.5253214596445318

After shifting the spectra, I was able to align the first two peaks close to reference values.[1] However, the relative positions are still slightly misaligned. I tested different CH_SIGMA values but this only changed the broadness of the peaks instead of its position.

Is there a parameter that I can change to reduce the amount of energy points so that the peaks become closer together? Or are my input files incorrect for calculating a molecule?

Thank you for your help,
Kevin.

[1]: Solomon, D., Lehmann, J., Kinyangi, J., Liang, B., Heymann, K., Dathe, L., Hanley, K., Wirick, S. and Jacobsen, C. (2009), Carbon (1s) NEXAFS Spectroscopy of Biogeochemically Relevant Reference Organic Compounds. Soil Sci. Soc. Am. J., 73: 1817-1830. https://doi.org/10.2136/sssaj2008.0228

Dear Kevin Tran,

VASP provides two different approaches for X-ray absorption spectroscopy (XAS):

• Supercell core-hole (SCH) calculations

• Bethe-Salpeter equation (BSE) for core excitations

There is a section on comparing SCH and BSE for XAS on the wiki, so I will not repeat that here in detail, but the gist of it is that you cannot expect SCH to correctly reproduce experimental data in most cases. So it is to be expected that your SCH results are slightly misaligned with respect to the experimental data.

I would thus suggest you switch to BSE for your XANEs calculation. It will be necessary to reduce your cell size to be computationally feasible, but that should not be a problem for this molecule.

According to our posting guidelines, please upload more information about your calculation, including relevant outputs and all input files, essentially a minimal reproducible example. Only then will we be able to assess your problem correctly and offer the best help.

Cheers, Michael