My Community

Hi,
I need to calculate the momentum matrix elements for my supercell.
I think getting a WAVEDAR file will do the job? but I am not sure would it include everything needed for using momentum matrix elements for optical properties?
I am confused in the potentials to use for getting a WAVEDER, as PAW doesnt include core of atoms and for momentum matrix elements and the optical properties we need cores? so should I use GW potential for calculating WAVEDER?
There are some post-processing tools to get (vasp2mat,vaspkit) who accurate are they to do this job ?

Hi,

You are correct. The WAVEDER file has the information you need. However, after looking at the documentation on the VASP wiki, it is unclear to me if this is still up-to-date. I will get back to you with a full answer once I have confirmation.

Hi again,

Sorry for the wait, but the situation is a bit more complicated than I originally thought. The WAVEDER file contains exactly what is written on the wiki. If that is what you want, meaning the derivative with respect to the Bloch vector, then you can use it. The derivative is already formulated in the PAW formalism, so it contains one-center terms. The only caveat is that the matrix elements corresponding to degenerate states, where \(\varepsilon{n \mathbf{k}} = \varepsilon{n' \mathbf{k}}\), are most likely meaningless. If they do contain values, those values should be ignored.

There are other ways to calculate this matrix in the code but none of them are currently accessible. If you have access to the code and are able to compile it, then you can perform a small modification that allows you to access a plane-text formatted version of the WAVEDER file called the WAVEDERF file, which should be easier to read. In linear_optics.F, there is a commented line

!   CALL WRT_CDER_BETWEEN_STATES_FORMATTED(WDES,W,IO%IU0,IU=55)

that can be uncommented and will then produce the WAVEDERF file.

If the matrix elements of the WAVEDER file is not what you are interested in, for example if you require the derivative with respect to the spatial coordinate, then there is currently no easy way to obtaining this matrix in VASP. We are working on making these matrices more accessible in future versions of the code.

I hope that this brings a bit more clarity to the situation.