VASP Forum

Hello everyone,
I am trying to calculate the optical properties of Al2O3 by GW+BSE. I already have the calculation result file(OUTCAR and vasprun.xml), but I am not sure where the relevant data is stored, so I would like to ask you for some information:
1. In which file is the data of transition energy stored? How to read the oscillator strength corresponding to the transition energy?
2. How to read the exciton binding energy?
3. How to read the dielectric function?

Hi Jike Wang,

There are a bunch of resources available that I can recommend for you to understand more about GW and BSE:

• Tutorials on GW

• Tutorials on BSE

• Wiki category page on GW (especially the How to section)

• Wiki category page on BSE (especially the How to section)

• A recently updated forum topic about plots showing oscillator strength and dielectric constants, discussing some pitfalls.

Now to your specific questions:

1. In which file is the data of transition energy stored? How to read the oscillator strength corresponding to the transition energy?

You want to search your vasprun.xml file for "opticaltransitions". There you will find two columns. The first one corresponds to BSE eigenvalues, the second to the oscillator strength

1. How to read the exciton binding energy?

You will have to subtract the BSE eigenvalues from the independent particle (IP) transition energies, both of which you can find in the BSEFATBAND file. A detailed tutorial on excitons analysis is available in exercise 10 of the BSE tutorials.

1. How to read the dielectric function?

The frequency-dependent dielectric function is again stored in the vasprun.xml file. Search for "dielectricfunction" to find the tensor data. They are right below the optical transition data. The layout is: "energy, xx, yy, zz, xy, yz,zx"
Exercise 9.2 of the BSE tutorials explains how to compute the dielectric function. There also are some scripts that could help you with plotting.

Please let me know if this answers your questions,
Michael

Dear Michael,

Thank you for your prompt response! However, I have a few questions regarding the BSEFATBAND file. I have carefully read Exercise 10 and learned that the exciton banding energy can be obtained from the IP energy and the BSE energy. However, I am still unclear about the exact meanings of these two values. Does the IP energy refer to the bandgap energy, and does the BSE energy refer to the exciton energy?

Best regards,

Jike Wang

Dear Jike Wang,

I apologize for the very late response; something went wrong with my email notifications.

I hope you figured things out, but just for completeness:

The IP energy is the energy difference between the conduction and the valence state involved in the specific transition, i.e. the eck - e_vk in the BSE equation. The exciton energy is the eigenvalue of the BSE Hamiltonian.

Maybe you would profit from watching two of our lectures, which are available on YouTube:
Optical Gaps
Dielectric Properties

All the best, Michael