HOMO/LUMO

[shank]

Dear sir
I have calculated HOMO and LUMO of a molecule using vnl package.Now i want to know, with what respect  the energy values are defined.In other words, what does correspond to 0eV in molecular energy spectrum.Does this represent vacuum level? I am learning the theoretical aspect of the molecular energy level.Please help.
Thanking you.
with best regards

[Nordland]

They are defined relative to the fermi level. So the energy E=0 is the fermi energy.

[shank]

Dear Nordland
Thank you.
with best regards
shank

[shank]

Just for clarification. If 0  level is Fermi level then LUMO will be always positive in molecular energy spectrum.But I found,LUMO is negative and is between HOMO and LUMO.

[Nordland]

Could you please share the output with me then it is easy for me to understand what the problem is

[Anders Blom]

Actually, and a bit confusingly, in ATK 2008.10 at least, the zero point for energy for molecular spectrum is not the Fermi level,  but the vacuum level, i.e. the value of the effective potential far away from the molecule. See http://quantumwise.com/documents/manuals/ATK-2008.10/ref.calculatemolecularenergyspectrum.html for reference.

[shank]

Dear Blom
Thanks for reply and link.I want to draw enegy level diagram considering HOMO and LUMO with Fermi level of metal contacts in used.

with best regards.
shank

[Anders Blom]

Right, so you had better adjust the values for the actual Fermi level. In the two-probe system, for transmission and MPSH eigenvalues, the zero level is adjusted to the (average) Fermi level of the electrodes.

[shank]

Thanks.
I want to study I-V characteristics in the device structure Si/molecule/Hg. The experimental data is very interesting, where  we observed sharp decrease in current near 1V and again current  increases with voltage.  We also did cyclic voltammetry of the deposited film,it resemblances with oxidation of molecule.Now i feel, under the bias, if molecule get oxidized, then its HOMO/LUMO will change and that will effect current flow. If it is so, how we will study the I-V using vnl.
with best regards