Subject: TDDFT--summary

From: "Shobe, Dave" <[email-masked]>
Date: Wed, 9 Apr 2003 11:53:28 -0400

Thanks to all who responded* to my questions on TDDFT.  I give a summary of
their responses below.

--Dave

*Tim Robinson, T. Daniel Crawford, Ulrike Salzner, Marcin Makowski,
'elewars', Stefan Grimme, Geoff Hutchinson.

++++++++++++++++++++++++++++++++++++++++++++++++++++

:: Time-dependent density functional theory (TDDFT) has become popular for
determining electronic 
:: excitation energies (approx. = lambda max in UV-visible spectroscopy) and
excited state properties.  
:: I know very little about this other than the name, but I would like to
learn.  So here are my questions.

:: 1. First, can one do TDDFT calculations using G98W?  There is a TD
command listed in the manual.  
:: I suppose that your route card would be something like this: 
::
:: # TD B3LYP 6-31G**  (?) 

This is the correct syntax, according to the responses.  You can also
specify NSTATES like you can with CIS.

:: 2. I normally use the hybrid functional B3LYP for ground state
calculations, as it seems to be one of 
:: the better functionals for that purpose.  But would a different
functional be recommended for TDDFT
:: calculations?

Most people said B3LYP was fine, although one response said that other
functionals perform somewhat better.  One responder mentioned
"asymptotically corrected functionals" which are unfortunately not included
in Gaussian 98 (maybe G03 will have them?).

Bibliography:
1. Hirata, Lee and Head-Gordon JCP 111 8904 (1999)
2. Bauernschmitt and Ahlrichs, CPL 256, 454 (1996)
3. K. B. Wiberg, R.E. Stratman, and M. J. Frisch, Chem Phys Lett, 1998, 297,
60
4. Head-Gordon et al. J Phys. Chem. A (2001) 105, p. 451.

:: 3. How good are the results compared to the CIS method?

TDDFT does not systematically overestimate transition energies the way CIS
does.   Also, it can describe types of excited states that CIS cannot
describe well.  If anything, the tendency is for TDDFT to underestimate
transition energies, especially for long conjugated pi systems.  ZINDO was
mentioned as a third alternative for organic systems.  [It would be nice if
ZINDO worked best with the conjugated pi systems that TDDFT has trouble
with, but no one has said this so far.]

Bibliography:
5. C. Diedrich S. Grimme, Systematic investigation of Modern Quantum
Chemical 
Methods to Predict Electronic Circular Dichroism Spectra, J. Phys. Chem. A, 
107, (2003), 2524-2539.
6. S. Grimme, M. Parac, Substantial errors from Time-dependent density 
functional theory for the calculation of excited states of large pi-systems,

ChemPhysChem., 4, (2003), 292-295.
7. J. Fabian et. al., THEOCHEM, 2002, 594, 41-53

:: 4. What caveats and common beginners' mistakes are there with TDDFT? 

I was advised to include diffuse functions in the basis set, and to set
NSTATES high enough to ensure the states of interest were all there.