Subject: HOMO-LUMO vs. lambda max : a summary

From: "Yeoh Hak Koon" <[email-masked]>
Date: Fri, 28 Apr 2000 15:38:16 +0800

Dear Colleagues,

The answers really came quick and unanimous from all over Western
hemisphere!  Thanks so much, especially to Stefan Grimme, Ulrike
Salzner, Alan Shusterman, Jens Spanget-Larsen, Wolfgang Roth, Deepak
Singh and Thomas Sommerfeld.  (Apologies : honorific excluded...).

Essentially, the major objection is that  the excited state responsible
for lambda max involves significant terms other than the HOMO-LUMO
energy difference (which is an orbital energy difference of ground
states), i.e. as manifested in the Coulomb and exchange integrals,
relaxation effects and electron correlation (configuration interaction,
CI) effects.  These can amount to a few eV, which is significant in a
UV-Vis spectra.

However, it was also pointed out that there is a possibility that the
contributions cancel out especially with the DFT.  But the results might
not be defensible as Koopman's theorem is not defined in DFT model.

I had actually tried out the comparisons with a commercial software on
about 30 molecules of all sorts...indeed the DFT can sometimes be real
close!  The semi-empirical models (PM3) are usually a few eV too high
compared to lambda max.  Curiously, the latter seems to be systemic,
i.e. I could add almost the same number of eV and get near to the lambda
max.  Does that ring a bell somewhere?

Hopefully I got the points right.  Thanks again.

Best regards,
Yeoh Hak Koon
28 April 2000