Subject: Is TD or Zindo better than CIS?

From: "Shobe, Dave" <[email-masked]>
Date: Mon, 22 Oct 2001 13:25:37 -0400

A question related to the last one. I'm aware that CIS has its shortcomings
and the results should be considered only a rough sketch of the molecule's
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?

I've noticed that the T.sub.e values are typically calculated at the same
level of theory as the ground state (using guess=alter I presume).  I
suppose this is preferable when it succeeds.  This is often possible for
diatomics, but once symmetry constraints are removed it's difficult to keep
the electron configuration from "falling" into the ground state.  

--David Shobe

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From: "Mark A. Thompson" <[email-masked]>
Date: Mon, 22 Oct 2001 13:28:55 -0700

Actually, they are the same thing.....as far as the equations used to
generate the CI matrix.

> this type of calculation?

Gaussian likely calculates excited states for Zindo and ab initio
Hamiltonians with a single-excitation CI (the Tamm-Dankoff approximation)
> from a singlet, single-reference ground state using a fixed geometry that
was used to generate the ground state SCF.  Certainly, more sophisticated
schemes are possible, such as including double excited configurations,
non-singlet ground states, etc.  I'm not sure if Gaussian supports these (at
least for the ZINDO Hamiltonian).

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From: Geoff Hutchison <[email-masked]>
Date: Mon, 22 Oct 2001 22:27:44 -0500

>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	<[email-masked]>
Ratner/Marks Groups	(847) 491-3295
Northwestern Chemistry	http://www.chem.nwu.edu/