Subject: How to compute LUMO eighen energy value by Gaussian or other programs in high accuracy?

From: "Kiniu WONG Kin-Yiu" <[email-masked]>
Date: Sun, 1 Oct 2000 20:56:26 +0800

Dear all CCLers,

    By Gaussian, we could get HOMO energy and LUMO energy by just looking at
the computed occupied eighen value and virtual unoccupied eighen value
respectively. In order to get values in higher accuracy, we could equate
HOMO to be E(cation) - E(neutral). Since HOMO energy means the minimum input
energy to the system in order to remove an electron from the system. So
during this process, the final state is just the cation of initial state.
With this approach, I could get a much better HOMO when compared with the
experimental value of the molecule (Alq3) in my project.

    However, on the other hand, it seems to be unreasonable to equate LUMO
to be E(neutral) - E(anion). If LUMO = E(neutral) - E(anion), then it
implies that LUMO energy means the minimum energy required to remove an
electron from the anion (the initial state). In other words, if LUMO =
E(neutral) - E(anion) then HOMO of anion = LUMO of neutral. We could imagine
it's the energy of the system loss to attract a free electron to the NEUTRAL
system.

    But, we also could imagine the HOMO-LUMO band gap in this way. Let E
equals net energy required to remove an electron from the system to infinity
then bring back this electron again to the system. However, this time the
electron should not be in the initial orbit. So the minimum of E is equal to
HOMO-LUMO band gap. If it's correct, LUMO is equal to the energy loss to
attract a free electron to the CATION of the system in an orbit other than
the initial one, such that E is the minimum. If it's right, then LUMO is not
E(neutral) - E(anion). It also could explain why the computed LUMO energies
are unreasonable in my project.

    So, my question is just that how to calculate LUMO accurately? Besides
just looking at the given eighen values, could we compute higher accuracy of
LUMO by Gaussian or other programs such as Jaguar?

    Thanks for any suggestions and helps!!

Regards,
Kiniu

------
Kiniu WONG Kin-Yiu
Undergraduate student
Department of Physics
The Chinese University of Hong Kong
Email: [email-masked]  or  [email-masked]