exciton wavefunction

[WEIW]

Dear developers,

It may occurs error when I try to calculate the wave function of excitons with negative energy. The exciton function does not converge, how can I correct this error? I want to know if this is the cause of negative energy.

I have attached the relevant file below.

Best,
Liu

[Daniele Varsano]

Dear LIU,

this is not related to negative excitation energy but to the fact that the wfs is localized in k space, essentially gamma point only (see weight output).
It is also possible that your k points sampling is not enough to localize it in real space.
My suggestion is:
1) Increase Degen_Step in order to consider a linear combination of degenerate excitons (your exciton is 4-fold degenerate).
2) Increase the number of cells, but lowering the FFTGvecs as otherwise the calculation could be very intensive.

Even so, if the weight is localized to just one k point, in real space it will be delocalized.

Best,
Daniele