how to know the HOMO & LUMO

[vboyz]

how to calculated HOMO & LUMO in both molecule and two probe system? which function should I use?
and how to know which are HOMO or LUMO in so many eigenfunction orbitals ?
Thanks!

[Anders Blom]

For a molecule, the easiest way is to evaluate the occupations using calculateEigenstateOccupations(). Then the HOMO is the highest level with non-zero occupation, and LUMO the next higher.

In a two-probe system I assume you are referring to the eigenstates of the MPSH; see more in this post.

[vboyz]

Thanks for your reply!
Now I have calculated the Occupations for a molecule.the result is as follow:
Level   Energy (eV)     Occupation
0         -24.06              2
1         -23.92              2
…………
550         -4.214         0.1975
551         -3.724      1.31e-009
…………
1239          14.99     9.881e-324
1240             15              0
……
which level is the HOMO?  550? or 1239 It's hard to decide for me ? Or another level you'll suggest.
and i have attached the out log.
thank you for your kind support!

[Anders Blom]

If you look in a bit broader interval, you will find

534         -4.718              2
535         -4.589              2
536         -4.588              2
537         -4.419          1.994
538         -4.412          1.991
539         -4.403          1.988
540         -4.396          1.984
541          -4.34          1.873
542          -4.34           1.87
543         -4.303          1.555
544         -4.299          1.493
545          -4.27         0.9783
546         -4.269         0.9705
547         -4.239         0.4515
548         -4.238         0.4459
549         -4.215         0.2082
550         -4.214         0.1975
551         -3.724      1.31e-009
552         -3.707     6.718e-010
553         -3.705     6.301e-010
554         -3.702     5.599e-010
555         -3.628     3.138e-011
556         -3.621     2.392e-011

While the population on 551 is not zero numerically (the Fermi function never goes to identically zero until infinity), it is for all practical purposes zero. Thus your HOMO is 550, but note that there are two almost degenerate levels at the same energy (549 and 550)!

[rahulprajesh]

but sir what about LUMO...?
which level is LUMO...(I'm a beginner sir so please dont mind...)

[zh]

LUMO: lowest unoccupied molecular orbital.
More details can be found here:http://en.wikipedia.org/wiki/HOMO/LUMO

[rahulprajesh]

I m not concern with the acronym of lumo....
I want to know the value of energy for LUMO for the above discussion....
like the HOMO level is 550 then which level should be the lumo ...and how
thanks...

[Anders Blom]

LUMO is by definition the level just above the HOMO.
So, 551 in the example above.