Re: CCL: Oscillator strength in TD-DFT

$)C
Here's my thoughts and I am eager to hear other views.  Zero oscillator strength can mean a variety of things.
 
(1)        You will have f=0.0000 for a spin-forbidden singlet-triplet transition.  In this case, there would be no experimentally observed transition.

(2)        For spin-allowed singlet-singlet transitions you can have f=0.0000 reported for a symmetry forbidden transition.  However, there may be an experimental peak due to vibronic couplings in this case.  A TDDFT calculation in Gaussian via the TD keyword does not include vibronic couplings; the proper treatment of vibronic coupling is really quite difficult.  
        A good example is for Formaldehyde.  The UV spectrum of CH3CHO has a single broad absorption peak at 326nm and the upper state is designated as 1A2(n,pi*): under electric dipole rules this singlet-singlet transition is forbidden.  That this transition is, in fact, experimentally observed is due to vibronic interaction as discussed by Dieke and Kistiakowski, Phys. Rev., 45, 4, (1934).

(3)        Finally, Gaussian will report f=0.0000 if f is < 0.0001



"CCL" <owner-chemistry]![ccl.net>
Sent by: owner-chemistry]![ccl.net

07/13/2005 08:23 AM

To
Robert L Waterland/AE/DuPont]![DuPont
cc
Subject
CCL: Oscillator strength in TD-DFT





Hello,
 
I would like to know about the meaning of zero oscillator strength, f=0.0000, in TD-DFT calculation.
Does it mean that the transition could not be observed in experiment ?
Any information or references would be greatly appreaciated.
 
 
Kind Regards,
 
Jangbae
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Jangbae Kim
Ph. D. Candidate
Nano/Bio Structural Dynamics Lab.
Department of Chemistry
Korea Advanced Institute of Science & Technology (KAIST)
373-1,  Guseong-Dong, Yuseong-Ku, Taejeon, 305-701, Korea
E-mail : biomimic]![kaist.ac.kr
TEL. : +82-42-869-2884
FAX. : +82-42-869-2810
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