Can exciton wavefunction plotted via ypp be periodic?

[xueshanxihua]

Dear all:

I'm using "ypp -e w" to plot the exciton wavefunction with a finished G0W0-BSE result. The hole position has been fixed by considering the orbital component of VBM. However, when setting the supercell to be extremely large, I find that the exciton wavefunction comes out to be periodic in real space, i.e., locally the wavefunction is still an envelope with finite size, but these "localized" envelopes repeat periodically in a larger scale.

So can anyone tell me how this happens? Have I done something wrong?

Many thanks~!

[Daniele Varsano]

Dear Zeyu,
Nothing wrong, Yambo is a plane wave code.
if you look the expression of the excitonic wave function in real space as obtained by FFT, you realize that it is periodic after a distance given by 1/deltaK, where deltaK is the smaller K-K' available in your sampling. If you see that the exciton wave function touch the border of your build supercell given by Nk*Lside, it means that probably your calculation is not converged with respect your kpoint grid
and you need a larger k point grid.

Best,

Daniele