Subject: CCL:How to compute LUMO eighen energy value by Gaussian or

From: "Jens Spanget-Larsen" <[email-masked]>
Date: Mon, 2 Oct 2000 12:00:59 +0100

Kiniu WONG Kin-Yiu:

>     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.  ..........

Dear Kiniu,

I am somewhat confused by your question. You wish to compute accurate HOMO and 
LUMO energies.  Molecular orbitals (MOs) are matematical functions defined 
within the so-called independent-particle model, and in order to compute them 
with "high accuracy" you would need to solve this model accurately.  But 
apparently, this is not really what you want to do?  What you want is probably 
accurate computation of ionization energies and similar quantities, but this is 
not the same as accurate computation of model quantities like MO energies. You 
state that "HOMO energy means the minimum input energy to the system in order 
to remove an electron from the system".  But this is only an approximation 
(usually called Koopmans' approximation) and does not in general lead to an 
accurate prediction of the ionization energy.  

Yours, Jens >--<
                             
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JENS SPANGET-LARSEN         Phone:  +45 4674 2000  (RUC)
Department of Chemistry             +45 4674 2710  (direct)
Roskilde University (RUC)   Fax:    +45 4674 3011 
P.O.Box 260                 E-Mail: [email-masked]
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