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<p>Dear Mohammed,</p>
<p><br>
</p>
<p>It is all about which spectroscopy you are interested in. Let's distinguish between absorption spectroscopy (ABS) and electron energy loss spectroscopy (EELS). In the following, let me explain the things how I understand them (based on my experience and
knowledge). In 3D, the situation is the following:</p>
<p><br>
</p>
<p>- ABS: TDDFT in the adiabatic local density approximation (ALDA) is OK for finite systems (e.g. molecules), but it is not OK for periodic systems (e.g. bulk silicon);</p>
<p>- EELS: <span>TDDFT</span> in ALDA is OK for periodic systems. I do not know much about EELS in finite systems, so I will not comment on that.</p>
<p><br>
</p>
<p>turboLanczos and turboDavidson are the codes for ABS in finite systems (not applicable to periodic systems).</p>
<p>turboEELS is the code for EELS in periodic systems.<br>
</p>
<p><br>
</p>
<p>Now, my "OK" and "not OK" above have to be clarified. For example, TDDFT with ALDA gives pretty good results for ABS in finite systems, but if you use it with hybrid functionals you will get better results. TDDFT with ALDA gives bad results for ABS in bulk
silicon (see e.g. 10 in Onida et al., Rev. Mod. Phys. 74, 601 (2002)). To compute ABS in periodic systems one typically uses the BSE approach (e.g. with the Yambo code which is interfaced with QE, but there are many other codes around). EELS in periodic systems
contains two types of excitations: collective (plasmons) - which are described well within TDDFT -ALDA, and single-particle excitations which are described rather well within TDDFT-ALDA. In general, this topic is quite complicated and I recommend to you to
read some papers/books e.g. by Lucia Reining and coworkers (e.g. the recent book by Martin, Reining, and Ceperley "Interacting Electrons" and the aforementioned RMP).
<br>
</p>
<p><br>
</p>
<p>Last, spectroscopy in 2D is yet another story, I am not an expect in it, so better I do not comment on that. I simply suggest to check literature and see what methods people typically use for accurate modelling of ABS and EELS in 2D.<br>
</p>
<p><br>
</p>
<p>HTH</p>
<p><br>
</p>
<p>Iurii<br>
</p>
<p><br>
</p>
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<font size="3" face="'Times New Roman', Times, serif" color="808080">--<br>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> users <users-bounces@lists.quantum-espresso.org> on behalf of Mohamed Ahmed Abd-Elati <maa@niles.edu.eg><br>
<b>Sent:</b> Tuesday, February 5, 2019 11:48:03 AM<br>
<b>To:</b> users@lists.quantum-espresso.org<br>
<b>Subject:</b> [QE-users] TDDFT for periodic structure</font>
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<div>Dear all <br>
</div>
<div>I am confusing about the calculation of TDDFT. As I read turbo_lanczos is used for isolated structure ( non periodic). However Turbo-EELS is used for bulk periodic structure. my question is how can I use TDDFT for two dimensional systems periodic structures
( graphene and BN sheets). Is that available in Quantum esspresso or not?<br>
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