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Re: Need for fermi level correction to compare LUMO

Alex Granovsky
gran@classic.chem.msu.su


Hello,

the answers are:

>1. Is there any chance for me to compare LUMO of different molecules?
Yes it is.

>2. Is this problem always appeared from DFT when we calculate molecular system with conjugation? Because I found some papers were published for that negative LUMO for the same type of my molecules.

Negative LUMO energies are quite typical esp. for DFT.

>3. Is Firefly make any absolute value for Fermi level?

the absolute zero Fermi level corresponds to zero orbital energy.

This can be explained as follows.

Just imagine that you have computed two molecules at a very large
separations in a single run i.e. as a super-molecule. The orbital
energies of each monomer will the be the same as those obtained
in separate calculations for individual monomers. This means that
there exists the common energy scale for two monomers; more
precisely this means that the orbital energy scale and origin is
the same for any molecules, and that you can compare orbital
energies.  

Kind regards,
Alex Granovsky



On Thu May 8 '14 3:48pm, pejer wrote
------------------------------------
>Dear Firefly user,

>I have problem with my LUMO energy. I calculate its energy (with DFT) of some molecules and obtain its HOMO and LUMO, and found that the LUMOs have negative values. I compared LUMO of one molecule with the other molecule in order to know whether electron can flow from LUMO molecule 1 to LUMO molecule 2. But, I couldn't find whether the absolute Fermi level of each molecule are same or not. In my point of view, if the Fermi level of each molecules are not same, I couldn't make any comparison.

>My questions are:
>1. Is there any chance for me to compare LUMO of different molecules?
>2. Is this problem always appeared from DFT when we calculate molecular system with conjugation? Because I found some papers were published for that negative LUMO for the same type of my molecules.
>3. Is Firefly make any absolute value for Fermi level? If no,
>4. Is there any correction for this?

>I would really appreciate for any help.
>
>
>Best regards

[ This message was edited on Mon May 12 '14 at 2:04am by the author ]


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