[Pw_forum] Turbo TDDFT for a quantum dot

Timrov Iurii iurii.timrov at epfl.ch
Mon Jul 25 10:02:17 CEST 2016

Dear Sunghwan and Dario,

I confirm Dario's guess. From a technical point of view, current implementation of turboTDDFT does not allow to use smearing. There are no conceptual problems to extend turboTDDFT such that to be able to treat systems with a smearing, but it has not been implemented yet.

Best regards,

Iurii

--

Dr. Iurii Timrov
Postdoctoral Researcher
Swiss Federal Institute of Technology Lausanne (EPFL)
Laboratory of Theory and Simulation of Materials (THEOS)
CH-1015 Lausanne, Switzerland

________________________________
From: pw_forum-bounces at pwscf.org <pw_forum-bounces at pwscf.org> on behalf of dario rocca <roccad at gmail.com>
Sent: Sunday, July 24, 2016 10:52 PM
To: PWSCF Forum
Subject: Re: [Pw_forum] Turbo TDDFT for a quantum dot

Dear Sunghwan,
That's my guess. If you set occupations='fixed' (the default) for the ground state calculation, the turboTDDFT will not complain that your system is metallic.
If your system has a 0.5 eV gap there will not be any problem. If the gap goes below 0.1 eV it migh be hard or impossible to converge the ground state calculation with  occupations='fixed'. In that case the turboTDDFT cannot be used.
According to me as long as the ground state energy converges with occupations='fixed' the turboTDDFT will run smoothly and give reasonable results for your nanoparticles.
Once again this is my guess, you should try.
Best,
Dario

On Sat, Jul 23, 2016 at 6:45 PM, SungHwan Choi <sunghwanchoi91 at gmail.com<mailto:sunghwanchoi91 at gmail.com>> wrote:
Dear Dario,
Thank you for a kind reply. You mean that if it is converged without smearing option, then there is no problem on doing turboTDDF calculation. Am I correct?
Sincerely,
Sunghwan Choi

========================================
Sunghwan Choi
Ph.D candidate
Computational Quantum Molecular Science Lab
Department of Chemistry, KAIST, Republic of Korea

Dear Sunghwan,
I have not used the code on systems with the characteristics you describe. This is my guess: As far as the ground state of your quantum dot converges without a smearing the turboTDDFT code can be used and will be stable. While the independent electron polarizability of a 0 gap system diverges for \omega-->0, the Hartree part in the TDDFT kernel will increase the value of the gap and/or will "push" the oscillator strength to higher energies.
Best,
Dario

On Fri, Jul 22, 2016 at 6:37 PM, SungHwan Choi <sunghwanchoi91 at gmail.com<mailto:sunghwanchoi91 at gmail.com>> wrote:
Dear all,
I am checking whether turboTDDFT program is suitable for my project. My project is to calculate absorption spectrum of metallic quantum dot. I have two questions.

1) From other post, I read the fact that turboTDDFT cannot calculate a metallic system. Here, how do we define metallic system? just system that have no band gap? then for small metallic quantum dot can be a non-metallic system since it has small band gap (0.1~0.5eV). case that fractional occupation does not happen? then, we can control the occupation parameter not to make fractional occupation.

2) A metallic quantum dot has a size-dependency for band gap. Thus, if size become larger, then band gap become narrow. When turboTDDFT failed to predict an absorption spectrum? and do you know a numerical trend of artifact, which come from low band gap?

By the way, does this limit come from algorithm or implementation of turboTDDFT? As far as I understood there is no limit of algorithm itself. I think there is some practical issues on there. Would you tell me where some troubles come up?

Sincerely
Sunghwan Choi

========================================
Sunghwan Choi
Ph.D candidate
Computational Quantum Molecular Science Lab
Department of Chemistry, KAIST, Republic of Korea

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