Re: CCL:Is TD or Zindo better than CIS?
At 1:25 PM -0400 10/22/01, Shobe, Dave wrote:
spectrum. Gaussian has two other methods (neither of which I've used) for
calculating excited states: Zindo and TD. Are these significatnly better
than CIS? Zindo being a semiempirical method, is it only good for certain
kinds of molecules? Should I be looking at programs other than Gaussian for
this type of calculation?
It's strange this should come up as I'm working on a manuscript along
these lines for a certain set of molecules. But first there's a
nomenclature issue. Normally when people say "Zindo," they
mean
"INDO/S CIS," and so you can't compare "Zindo" to
"CIS." Also, I'm
not quite sure I know what you mean when you say "TD" since
you
usually refer to TD-HF or TD-DFT for time-dependent HF or time
dependent DFT.
If you compare Zindo CIS to HF-CIS for most neutral organic
molecules, you'll find Zindo is more accurate. If you compare TD-HF
(say the RPA) to HF-CIS, the HF-RPA is better--no surprise since the
RPA is one step past the Tamm-Dancoff approximation (CIS). But HF-RPA
is not as good as Zindo for most neutral organic molecules.
TD-DFT seems to be slightly worse than Zindo for neutral organic
molecules, but not by much.
Also keep in mind that Zindo CIS as a semiempirical method sometimes
incorrectly identifies the composition of certain excitations even if
the energy is pretty good.
Can't say how Gaussian does on these sorts of things--we can't get it
here. As Mark Thompson pointed out, you can also go beyond CIS to
CISD, etc. The original ZINDO program from Zerner does let you do
things like this, though YMMV. Other programs as well have CISD,
CISDT and other higher-order methods. (Dalton springs to mind.)
--
--
-Geoff Hutchison <hutchisn-0at0-chem.nwu.edu>
Ratner/Marks Groups (847) 491-3295
Northwestern Chemistry http://www.chem.nwu.edu/