Subject: Summary: TDDFT question

From: "Deng, Jun" <[email-masked]>
Date: Mon, 12 Aug 2002 09:34:40 -0400

Thanks all for replying to the TDDFT question, following is the summary:

Original message

> Does anyone has experience in TDDFT calculation on a charged molecule?
> I found that for neutral compounds, TDDFT generally gives smaller 
> excitation
> energy than experiment (but good correlation).  However, for charged
> molecules, it gives much, much higher energy and no correlation.  Why?  
> Any suggestion?

1) From Gaussian

Dear Dr. Deng,
     I'm not aware of any systematic reason as to why TD-DFT would 
perform poorly for charged systems. The lack of correlation with 
experiment could be due to a number of reasons. This could be a case in 
which the functional or basis set used in not doing a very good job of 
describing the ground state of the charged system, and therefore the 
TD-DFT excited states are unreliable. Depending on whether your systems 
are positively or negatively charged, the basis set effects can be 
increased with respect to the calculations on the neutral systems. 
Additionally, if the excited state energies are much higher for the 
ionic systems than they are for the neutral species, this could be a 
reason that TD-DFT is doing a systematically poorer job on the charged 
species you are investigating. While it does not deal with charged 
systems, the following paper about basis set effects might be of 
interest to you:

"A Comparison of the Electronic Transition Energies for Ethene, 
Isobutene, Formaldehyde, and Acetone Calculated Using RPA, TDDFT, and 
EOM-CCSD. Effect of Basis Sets.", K.B. Wiberg, A.E. de Oliveira, and G. 
Trucks, J. Phys. Chem. A, Volume 106, Number 16, 2002.

Regards,
Jim Hess

++++++++++++++++++++++++++
James Hess, Ph.D.
Customer Support Scientist
Gaussian, Inc.
e-mail: [email-masked]
++++++++++++++++++++++++++

2) From Dr. Egbert Zojer

I did play around a bit with ologophenylenes varying the chain length and I 
found theat for excitations within the singlet and triplet manifolds the 
experimental results are extremely well reproduced (A. Pogantsch et al. J. 
Chem. Phys., in print), while for charged molecules the results were worse than 
those I obtained a few years ago using semiempirical methods (even when 
including diffuse basis functions). This effect is not totally cler to me yet, 
but obviously there seem to be some complications for charged molecules and TD-
DFT (we are working on that)

regards,

egbert zojer
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Univ.-Doz. Dr. Egbert Zojer                             Tel.:++43/316/873-8475
Advanced Materials Division                             Fax.:++43/316/873-8478
Institute of Solid State Physics
Graz University of Technology                  e-mail.: [email-masked]
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3)From Hatice Can [[email-masked]]

I have done some TDDFT excited state calculations for
a large organic cation.My method was SVWN with 6-31G**
level basis set. I have the energies was smaller than
experimental but not too small. If I used the B3LYP
with the same basis set, I had smaller energies that
obtained with SVWN.
Which DFT function did you use? Perhaps, you may
change your dft function and may try higher basis set.

I hope it helps.when you receive another reponses, I
will be so glad, if you could summarise them.

Best regards,

Hatice