HOMO-LUMO vs. lambda max

["Yeoh Hak Koon" <yeohakoon ^at^ hotmail.com>]

Dear Colleagues,
 Something not addressed in all past postings :
 What are the reservations or objections of equating the HOMO-LUMO energy gap
 to the maximum wavelength peak (lambda max) in a UV-Vis spectra?
 Are these the reasons?
 (a) the HOMO-LUMO transition might be forbidden, hence does not show up in
 the spectra
 (b) the lambda max has vibrational contributions, so it is usually more
 energetic than the 0-0 transition predicted.
 For (a), is there a way to predict whether the transition is forbidden or
 not?  (I am using a commercial software PC Spartan Pro).
 For (b), since at room temperature, the vibrational energy is relatively
 small, I could still say the approximation is acceptable, right?
 My apologies if these insult your intelligence; I am not trained as a
 chemist, but a chemical engineer.  Thank you very much.
 Best regards,
 H. K. Yeoh
 26 April 2000