CCL: Queries on TDDFT calculations on emission

[Jamin Krinsky <krinsky.jamin^-^gmail.com>]

 Sent to CCL by: Jamin Krinsky [krinsky.jamin]=[gmail.com]
 Dear Eliz,
 It depends on which type of emission you want to model. Absorption
 will normally be only from the ground singlet state to various excited
 singlet states. Fluorescence occurs from the lowest singlet excited
 state to the ground state, but at the geometry of the first singlet
 excited state. So you would need to do a TDDFT optimization at the
 first singlet excited state, and the emission wavelength would be the
 excitation energy at the end of your optimization.
 If inter-system crossing can occur, the first singlet excited state
 can relax to the first triplet excited state. Because emission from
 this state to the ground state is spin forbidden the excited state
 will be longer-lived, and the emission process is phosphorescence. The
 cheapest (and pretty reasonable) way to calculate that emission is to
 do a geometry optimization on the "ground state" triplet using
 unrestricted, normal DFT, then do a TDDFT calculation from the singlet
 at that geometry and specify triplet excited states. Note that this
 last calculation will not give you oscillator strengths, just
 wavelengths.
 This basic discussion can be found in any good comp. chem. text, but
 if you are just starting out you should read up on when you can
 believe your results, for example:
 J. Chem. Phys. 2002, 117, 5543
 J. Am. Chem. Soc. 2004, 126, 4007
 J. Chem. Phys. 2008, 128, 044118
 Other people might have some better references for these things, this
 is just what I happen to have in front of me.
 Good luck,
 Jamin
 On Wed, Dec 15, 2010 at 2:45 AM, Eliz Lam elizabeth.shlam:_:gmail.com
 <owner-chemistry{=}ccl.net> wrote:
 >
 > Sent to CCL by: "Eliz  Lam" [elizabeth.shlam*|*gmail.com]
 > Dear CCLers,
 >
 > Hi, I'm a beginner in learning DFT and TDDFT calculations and have some
 queries on calculating emission properties of a molecule.  I originally
 thought the emission of a molecule should be the first triplet excited state
 from TDDFT calculations as stated from Kasha's rule that the emissive state
 should be of the lowest energy.  However, it seems not always the case.
  I'm wondering why one could not predict emission from the above way and
 how should one predict the emission of a molecule?  And is the emission
 mechanism originated the same as from first triplet excited state in the TDDFT
 calculations?
 >
 > Thank you so much!
 > Eliz>      http://www.ccl.net/cgi-bin/ccl/send_ccl_message>;  
    http://www.ccl.net/cgi-bin/ccl/send_ccl_message>;  
    http://www.ccl.net/chemistry/sub_unsub.shtml>;  
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 >
 >