[DFTB-Plus-User] Calculation of HOMO and LUMO levels

[Bálint Aradi]

The concepts you refer to make strictly only sense at T=0 K, when you 
have integer occupations only. If you have fractional occupations due to 
electronic temperature, you will have of course partially occupied 
states above the Fermi level and partially emptied states below that.

In general, fractional occupations can arrise for following reasons:

* Your HOMO is degenerated but not fully filled (then you would have 
partial occupations even at T = 0 K)

* You apply an electron smearing function (such as Fermi) using an 
electronic temperature, which is high enough to generate "excitations" 
in your system. If you have a metallic system, that happens at basically 
any finite temperatures, for a non-metallic one it obviously depends on 
the size of the gap.

Whether your system is metallic or non-metallic you can tell by looking 
and band.out. If it is supposed to be non-metallic, but you still have 
partial occupations then

* either, you made a mistake with your geometry, and the system you 
calculate is not the one you think to calculate, but a metallic one instead

* or your filling temperature is just too high for your gap.

Best regards,

Bálint


On 14.06.23 07:38, Hemanth Haridas wrote:
> Thank you for your response. I am trying to calculate the HOMO and LUMO 
> levels of a DNA fragment at Gamma point. As per your advice, I tried 
> reassigning the occupation numbers of the fractionally occupied states, 
> and found that the last fully occupied state would be lying nearly 5 
> states below the calculated Fermi level. However, my understanding is 
> that Fermi should be between the occupied and unoccupied levels for a 
> non-metallic system. Any pointers on this matter would be much appreciated.
> 
> Thank you for your time.
> 
> Yours sincerely,
> Hemanth
> 
> On Tue, Jun 13, 2023 at 11:44 AM Bálint Aradi <aradi at uni-bremen.de 
> <mailto:aradi at uni-bremen.de>> wrote:
> 
>      >
>      > Thank you for your reply. However when I checked the band.out
>     file, I
>      > have few energies with fractional occupancy. And they lie below the
>      > Fermi level.
> 
>     If you have fractional occupations such as for metallic systems, then
>     HOMO and LUMO are somewhat ill defined concepts. Probably, the
>     Fermi-level is the best thing you can have then. If you wanted, you
>     still could do define a HOMO, of course, by counting which would be the
>     highest occupied level, if you filled up all levels with integer number
>     of electrons until all electrons had been used up, but I am unsure,
>     whether that would be a very useful quantity.
> 
>     Best regards,
> 
>     Bálint
>      >
>      > On Tue, 13 Jun, 2023, 11:35 am Bálint Aradi, <aradi at uni-bremen.de
>     <mailto:aradi at uni-bremen.de>
>      > <mailto:aradi at uni-bremen.de <mailto:aradi at uni-bremen.de>>> wrote:
>      >
>      >     Dear Hemanth,
>      >
>      >     HOMO = highest occupied molecular orbital, LUMO = lowest
>     unoccupied
>      >     molelular orbital. Occupations of the molecular orbitals can be
>      >     found in
>      >     the band.out file. Both levels should be in the vicinity of the
>      >     Fermi-level.
>      >
>      >     Best regards,
>      >
>      >     Bálint
>      >
>      >
>      >     On 13.06.23 07:18, Hemanth Haridas wrote:
>      >      > Dear all,
>      >      >
>      >      > I am trying to calculate the HOMO and LUMO levels of a
>     molecule
>      >     using
>      >      > DTB+. However, I could only find Fermi energy in the
>     detailed.out
>      >     file.
>      >      > The calculation was performed on the Gamma point. Any
>     pointers on
>      >     how to
>      >      > calculate HOMO and LUMO levels would be much appreciated.
>      >      >
>      >      > --
>      >      > *Hemanth H 18310019*
>      >      > Research Scholar
>      >      > IIT Gandhinagar
>      >      >
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>      >     --
>      >     Dr. Bálint Aradi
>      >     Bremen Center for Computational Materials Science, University
>     of Bremen
>      > http://www.bccms.uni-bremen.de/cms/people/b-aradi/
>     <http://www.bccms.uni-bremen.de/cms/people/b-aradi/>
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>     -- 
>     Dr. Bálint Aradi
>     Bremen Center for Computational Materials Science, University of Bremen
>     http://www.bccms.uni-bremen.de/cms/people/b-aradi/
>     <http://www.bccms.uni-bremen.de/cms/people/b-aradi/>
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> 
> -- 
> *Hemanth H 18310019*
> Research Scholar
> IIT Gandhinagar
> 
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-- 
Dr. Bálint Aradi
Bremen Center for Computational Materials Science, University of Bremen
http://www.bccms.uni-bremen.de/cms/people/b-aradi/


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