<html><body><div style="font-family: arial,helvetica,sans-serif; font-size: 12pt; color: #000000"><div>Hi.<br></div><div><br></div><div>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.<br></div><div>Do you need a full excitonic optical spectrum ? or just a RPA spectrum is sufficient for your study?<br></div><div><br></div><div>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.<br></div><div><br></div><div>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.<br></div><div><br></div><div>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.<br></div><div><br></div><div>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 .<br></div><div>I would reccomend especially<br></div><div>Onida Reining Rubio Rev Mod Phys 2002 <a href="http://journals.aps.org/rmp/abstract/10.1103/RevModPhys.74.601">http://journals.aps.org/rmp/abstract/10.1103/RevModPhys.74.601</a><br></div><div>as all the others references in the yambo website as well.<br></div><div><br></div><div>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.<br></div><div><br></div><div>I hope this could help a bit.<br></div><div><br></div><div>With the best<br></div><div>Federico<br></div><div><br></div><div><br></div><div><br></div><div><span name="x"></span><table class="mceItemTable" xmlns="urn:zimbraAccount"><tbody><tr><td><img alt="Université Paris-Sud" src="https://zimbra.u-psud.fr/public/z_logo_signature.png" style="border: 0pt none;width:120px;height:107px;background:url(data:image/png;base64,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" data-mce-src="https://zimbra.u-psud.fr/public/z_logo_signature.png" data-mce-style="border: 0pt none; width: 120px; height: 107px; background: url(data:image/png;"></td><td valign="top"><div><span style="font-size: 10pt" data-mce-style="font-size: 10pt;"><b>Federico IORI</b></span></div><div><span style="font-size: 10pt" data-mce-style="font-size: 10pt;">Marie Curie Fellow<br>Laboratoire de Physique des Solides<br>Bâtiment 510 - Rue André Rivière<br>91400 Orsay</span></div></td></tr></tbody></table><div><br></div><span name="x"></span><br></div><hr id="zwchr"><div style="color:#000;font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt;" data-mce-style="color: #000; font-weight: normal; font-style: normal; text-decoration: none; font-family: Helvetica,Arial,sans-serif; font-size: 12pt;"><b>De: </b>"Muhammad Adnan Saqlain" <saqlain@bilkent.edu.tr><br><b>Para: </b>"pw forum" <pw_forum@pwscf.org><br><b>Enviados: </b>Sábado, 17 de Septiembre 2016 13:25:12<br><b>Asunto: </b>[Pw_forum] calculation of absorption spectrum<br><div><br></div>Dear users<br><div><br></div>I have a system consisting of 94 atoms (periodic with cell lengths of <br>16.5, 8.5 and 8.7). I want to calculate its absorption spectrum. Can <br>anyone guide me which technique would be easy to run. Actually, we have <br>16 core machine and we can run a job for a maximum of 15 days only. <br>Based upon this time limit, I would like to know which technique would <br>be suitable. TDDFT or GW?<br><div><br></div>any help in this regards would be highly appreciated.<br><div><br></div>Thanks<br><div><br></div>Adnan<br><div><br></div>Bilknet U. Ankara<br><div><br></div>_______________________________________________<br>Pw_forum mailing list<br>Pw_forum@pwscf.org<br>http://pwscf.org/mailman/listinfo/pw_forum<br></div><div><br></div></div></body></html>