RPA_LF

[kxzhou]

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

[Daniele Varsano]

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

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.

Best,
Daniele

[kxzhou]

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

[Daniele Varsano]

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