[Pw_forum] Turbo TDDFT for a quantum dot

Sun Jul 24 22:52:40 CEST 2016

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
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>
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
>
> 2016-07-23 16:43 GMT+09:00 dario rocca <roccad at gmail.com>:
>
>> 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>
>> 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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>>
>>
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