From owner-chemistry@ccl.net Wed Dec 15 13:55:01 2010 From: "Jamin Krinsky krinsky.jamin%a%gmail.com" To: CCL Subject: CCL: Queries on TDDFT calculations on emission Message-Id: <-43399-101215135031-25109-AwP2KVe/uAAM4OonLZGooA#,#server.ccl.net> X-Original-From: Jamin Krinsky Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=ISO-8859-1 Date: Wed, 15 Dec 2010 10:50:01 -0800 MIME-Version: 1.0 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 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>      http://www.ccl.net/spammers.txt> > >