Yambo Community Forum

Dear Sahu

3.You need just the q=(0,0,0) because you are looking at optical transitions.
As I said, o.eps_q001-rpa contains the optical absortion spectrum calculated within the RPA. When comparing with experiment remember RPA is not a good approximation for the absorption spectra of for semiconductors and insulators (see discussion in sec 2.2 of the Yambo paper)
NB note that q=(0,0,0) is always present

4. For each \omega the calculated eps_M is a complex number, and there are built in functions to get its real and imaginary part

5. Eq. 14 is the definition for eps_M as function of eps-1 irrespective of considering or not LF. LF are "in" eps-1 since you considered nondiagonal elements of X_0 in eq. 7. The variable NGsBlkXd refer to the block size of the response function X_0.

Given the medium (this forum) I am afraid I cannot be much more clearer (well maybe others can). If you have still doubts you can probably refer to the works cited in the yambo paper where things may be explained in some more detail. See e.g. ref 2.

Cheers

m

brsahu wrote: About (3): For each transferred momenta i.e q' s in yambo.in, yambo -o c write one file including q= (0,0,0) point, if this point is in the q-list. My question was to compare with experimental absorption spectra, do one use only the file for q=(0,0,0) and plot second versus the first column in o.eps_q001-rpa? Also only q=(0,0,0) is considered because the contribution to absorption spectra is the dominant one compared to other q's in the list?

Dear shau,

be so kind to permit me to turn your questions back to you. What is an absorption spectrum ? What is the relationship between the momentum (q in Yambo) and a typical absorption experiment ? Once you will have answered all these questions all the machinery used by Yambo will be much more clear. If you need help please have a look at the Yambo paper and at the references used therein.

brsahu wrote: About (4): I was asking how one separates explicitly the real and imaginary part of the macroscopic dielectric constant after solving equation (14), I mean how the code does it. It is clear from your previous mails which columns in the file corresponds to the re/im part of it.

This separation is performed in O_driver.F in the directory src/pol_function of the code source. Have a look yourself. It is not difficult, once the full complex dielectric constant is calculated (X_epsilon if I remember correctly) then you extract real and imaginary part by using simple fortran commands (real, aimag ...).

brsahu wrote: About (5): I mean if G=G'=0 is the limit taken while calculating equation (14), then are we including the local field effects in the spatial average of the macroscopic dielectric constant because only G=G'=0 is only one component.

Again, you can easily answer this question by having a look, for example, ref. 2 of the Yambo paper.

Dear Mryta and Andrea,

Thanks for the replies.

Probably I stretched the discussion too far. Nevertheless, it gave an opportunity to understand some details in addition to the available literature.

BTW Mryta your paper on exciton-plasmon is interesting.

Sahu

brsahu wrote: Probably I stretched the discussion too far. Nevertheless, it gave an opportunity to understand some details in addition to the available literature.

Dear Shau,

please do not feel sorry It is normal to have tons of questions about a code that you just started using. Simply the message we wanted to give is that you must not fall in the temptation of learning the theory of linear response (or anything else) by running or looking inside the code. Yambo is a tool and to use it properly you should first learn the theory that is behind.

If users will keep making a lot of questions about the theory we may open a dedicated subforum. Let's see

Best regards ! And keep using Yambo and keep us informed about your progresses