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.