Yambo Community Forum

Dear friends,

While analyzing the excitonic function, I know there are two ways to plot the electron density: one is "ypp -e w", which fixes the hole position and calculates the norm of excitonic wavefunction. I have no problem with this; the other is "ypp -e w -f", which allows the hole to move freely according to the explanation of "ypp -H". Does it mean that the excitonic wave function is integrated over the position of hole, before calculating its norm?

I am a little confused because in the tutorial (http://www.yambo-code.org/tutorials/hyd ... /plots.php) the other way to plot the wavefunction is stated as "electron and the hole are forced to be in the same space point".

Could you please elaborate the exact meaning of "ypp -e w -f"?

Dear Guangfu Luo,
excitonic wfs is a two variable function, i.e. is a function of position of electron
and the position of the hole. ypp -e w permits you to set one of the two variable (the hole position)
to a fixed position in the space. The free hole option means that the hole position is not fixed, but
it has the same vaule of the electron position.
In one case you have Psi(r_e,r_h) r_h fixed to r_0.
In the other situation Psi(r_e,r_h) with r_h=r_e

Hope it helps.

Daniele

Dear Daniele,

Thanks for your explanation; now I get their exact meanings. So, fixing hole position is more close to the physical picture, but obviously it is still not good enough.

According to some suggestion in this forum, we choose the special hole position according to the density of valence band which contributes to the exciton. While doing this, I find sometimes there are several equally important choices, and they give different exciton distribution. Therefore, none of them alone represents the whole picture.

Now I'd like to propose this question: why do not we consider this issue stricter and include all possibilities in space? We just need to integrate the excitonic wavefunction over the hole position. Is this difficult or do I miss something (since all the papers I know did not do such thing)?

Dear Guangfu Luo,

the way you suggest could bring more physical insight. Another way could be to transform
the electron and hole position in they center of mass and relative position and integrate
in the center of mass position. We still not have done this, essentially for lack of time, and
because fixing the hole position in a "smart" way, in most of the case is enough to extract
the physical character. If you have time and will to implement this stuff you are more than
welcome.Of course we can help you in the task.

Cheers,

Daniele

Dear Daniele,

Thanks for your encouragement. Actually, I will have some free time in the near future, and I'd like to have a try (though I have not use Fortran for quite a long time.) Could you please let me know where I can find the necessary information to do this?

Sincerely,

Dear Guangfu Luo,

you can start having a look at the ./ypp/exciton_driver.F subroutine.
This is the subroutine where the excitonic wfs is calculated,
in particular you can find there the ##EXCITONIC WF SECTION##.

Cheers,

Daniele