Dear Federico

Thanks for your detailed response. I would look meet prof. Seymour Cahangirov. I actually, have recently joined Bilkent, so do no know many people yet.

On 09/17/2016 04:41 PM, Federico Iori wrote:

In my opinion, I think the approach you chose depends strictly from the question you want to answer and after that from the the size (number electrons, Number plane wave thou) you have. Do you need a full excitonic optical spectrum ? or just a RPA spectrum is sufficient for your study?

If it is silicon-like and then maybe not too much computationally demanding, I think you can use a linear response TDDFT as a first step and then move to a more sophisticated approach as GW (to calculate not the optic, but the screening of your material) + Bethe-Salpeter equation BSE to have correctly acces to the excitonic effects in your absorption spectra.

Of course all the calculations have to be converged wrt kpoints, number of Gvectors, number of bands. When increasing these parameters wrt their pristine value for your converged density in the DFT framework the computational cost will increase not linearly.

I would suggest you to have a look and to do the tutorial with the Yambo code on a small system as they proposed in order to correctly understand what are you doing.

Moreover there is the theory to have a look at in order to understand the meaning and the effect of the quantities you are calculating .
I would reccomend especially
Onida Reining Rubio Rev Mod Phys 2002
as all the others references in the yambo website as well.

Moreover, since I notice you are in Bilkent UNiv, try to ask to Seymour Cahangirov from the same univ. I think he can address you properly.

I hope this could help a bit.

With the best

Université Paris-Sud    
*Federico IORI*
Marie Curie Fellow
Laboratoire de Physique des Solides
Bâtiment 510 - Rue André Rivière
91400 Orsay

*De: *"Muhammad Adnan Saqlain" <>
*Para: *"pw forum" <>
*Enviados: *Sábado, 17 de Septiembre 2016 13:25:12
*Asunto: *[Pw_forum] calculation of absorption spectrum

Dear users

I have a system consisting of 94 atoms (periodic with cell lengths of
16.5, 8.5 and 8.7). I want to calculate its absorption spectrum. Can
anyone guide me which technique would be easy to run. Actually, we have
16 core machine and we can run a job for a maximum of 15 days only.
Based upon this time limit, I would like to know which technique would
be suitable. TDDFT or GW?

any help in this regards would be highly appreciated.



Bilknet U. Ankara

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