Yambo Community Forum

Dear all,
1) The dielectric function has been computed from DFT energies in GW calculations. Can this be used to calculate the absorption spectrum for RPA_LF? How should this be done?
2) Is it possible to use the ndb.qp obtained from GW calculations to compute the dielectric function for RPA_LF and thereby obtain the spectrum? Could this be challenging due to the need for an excessively dense k-point grid and a large number of quasi-particle bands?

Looking forward to your reply! Thanks!
Kexin

Dear Kexin,

1) No, as for the GW most probably you computed the dielectric screening at RPA level either using Plasmon Pole approximation (which requires just 2 frequencies) or MPA (limited number of frequencies). For a complete spectrum, you need to compute it for all the frequencies of the spectrum you are interested in.

2) Yes, it is possible using the keyword You do not need to calculated QP for all bands and k points. Yambo will read the calculated values and interpolate for the missing ones.

Best,
Daniele

Dear Daniele,

2) Yes, it is possible using the keyword

Code: Select all

XfnQPdb= "E < ./$DIR/ndb.QP"

You do not need to calculated QP for all bands and k points. Yambo will read the calculated values and interpolate for the missing ones.

Thanks for your reply!
For RPA_LF calculations, the number of BndsRnXd required can reach hundreds, but in G0W0, only the quasi-particle energies of two bands were calculated. Can interpolation still be performed under these conditions?
Can the parameters for RPA_LF, such as BndsRnXd, NGsBlkXd, and LongDrXd, be set the same as those for G0W0_ppa calculations?

Best,
Kexin

Dear Kexin,
1) if you calculated the GW corrections for just two values, the effect of the interpolation will be just a scissor operator, i.e. a rigid shift opening the gap. If a scissor approximation is valid, that's ok, otherwise you need to calculate the correction for more bands and k points.
I suggest you to calculate the corrections for few bands and different k points and verify if a scissor/stretching approximation can be applied. See e.g. step 3 of this tutorial: https://wiki.yambo-code.eu/wiki/index.p ... tandalone)

2) BndsRnXd: These now depends on the range of the spectrum you are interested in.
NGsBlkXd: You can use the value obtained to converge the GW corrections.
LongDrXd: This gives you the polarization of the incident light, so it depends on the direction you want to look at.


Please note that RPA_LF can also be obtained in the eh basis set, doing a BSE like calculation setting BSKmod="Hartree". In this way you do not need to set the number of frequencies as you have the spectrum. What way is more convenient depends on the system (bands, k points, dimensions of the screening).

Best,

Daniele