Subject: CCL:Absorption spectra from ZINDO/S

From: Charles & <[email-masked]>
Date: Tue, 14 Oct 1997 16:36:45 -0400

Dear CCLers,
	Following the discussion of the ZINDO/S interaction
factors...

	While it is not possible to explain from first principles
where the interaction factors come from in ZINDO, it is possibke
to obtain ab initio interaction factors for pi-electron
theory.  Since the ZINDO method was originally derived using
ideas from pi-electron theory, it may be helpful to understand exactly
how pi-electron theory works to understand how to modify the ZINDO
interactions.

(BTW, the original idea for adjusting the pi-pi interaction
factor in all-valence electron methods came from the work of 
Del Bene and Jaff'e (JCP vol 48, page 1807, 1968) - at least this
is the earliest reference I know of.   )

	It is possible to derive the pi-pi interaction factor in
semi-empirical pi-electron theory directly using Freed's ab initio
Effective Valence Shell Hamiltonian theory.  In particular, ths has
recently been performed on a series of small, protonated Schiff
bases in order to show how to properly reparameterize MNDO
for MNDO-piCI calculations.

	The work on the MNDO-piCI calculations is currently under
review, but you can find a preprint of it, along with some other
details about the Effective Hamiltonian Calculations, on the
Effective Hamiltonian Web Page (www.cat.syr.edu/~cmartin/hv).

	  Again, note that this work rigorously applies to
calculations using a pi-Full-CI or pi-SDCI and not the
ZINDO all-valence-electron SCI, but some of the general ideas may transfer.
For instance, in my opinion, it should be reasonble to adjust these
interaction factors for a series of related compounds.  By
realted compounds, however, I mean that the excited states of
these compounds should have similar character.  For example,
it might prove less usefull to use the same parameterization for
protonated Schiff bases as for neutral polyenes because
the excited states of neutral polyenes have much more ionic
character than the protonated Schiff bases.  (It may also be desirable
to modify the one-center,  two-electron Coloumb intergrals, as
indicated by the Effective Hamiltonian Theory).

	Chuck Martin

-- 
-------------------------------------------
Dr. Charles H. Martin
Quantum Theory Project
362 Williamson Hall
PO Box 118435
University of Florida
Gainesville, Florida  32611-8435

Fax: (352) 392-8722
Email: [email-masked]
Web:   www.cat.syr.edu/~cmartin/hv
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