Subject: Re: CCL:PPP Method: status?

From: "Jens Spanget-Larsen" <[email-masked]>
Date: Thu, 17 Feb 2000 14:28:36 +0100

E. Lewars:

> Subject: the Pariser-Parr-Pople method
> 
> Could someone with some familiarity with the PPP method please tell me:
> 1) is it still used at all?

Until quite recently, highly parametrized PPP models were used in the dye 
industry, see for instance the publications by J. Griffiths from the Dept. of
Colour Chemistry and Dyeing at Leeds University (UK): - "Practical aspects of
colour prediction of organic dye molecules", Dyes and Pigments, 1982, 3:211-233
- "Modern Dye Chemistry", Chemistry in Britain, 1986, 997-1000 - "Funktionelle
Farbstoffe", Chemie in unserer Zeit, 1993, 12:21-31

> 2) was it used for calculation of UV spectra *at a given geometry* only,
> or could it perform realistic geometry optimizations on pi-systems?

The classic PPP pi-electron method is a combination of Pariser and Parr's CI
model and Pople's SCF-MO model and was designed for prediction of pi-pi*
electronic transitions, assuming a 'given geometry'.  In order to 'perform
realistic geometry optimizations' you obviously need to consider explicitly the
role of the sigma-core.  Several hybrid models, describing the sigma-system by a
mechanical force-field model and the pi-system by a Pople-like SCF-MO method,
have been proposed for the prediction of geometries and heats of formation for
conjugated systems.  For an account of M.J.S. Dewar's work in this field, see
his book: "The Molecular Orbital Theory of Organic Chemistry", Chapter 5 ('The
Pople Method').  For modern implementations, see for example the treatise by
J.J. Gajewski, K.E. Gilbert, and J. McKelvey in "Advances in Molecular
Modeling", 1990, 2:65-92, and lit. cited therein.    

> 2) what has it evolved into: what currently popular method or program
> for calculating electronic spectra is an offspring of the PPP method?

J.A. Pople extended his pi-electron SCF-procedure to descriptions involving all
valence electrons, resulting, of course, in the all-valence NDO procedures CNDO,
INDO, etc.  Corresponding procedures CNDO/S and INDO/S were developed and
parametrized for the prediction of electronic transitions. Today, these
procedures have essentially replaced the classic PPP method. The most popular
model in this field is probably ZINDO/S, developed by the late M.C. Zerner and
his group in Florida; ZINDO/S is included, for example, in the current versions
of HyperChem and Gaussian.

Yours, Jens >--<

PS. One beauty of the standard PPP method is the presence in this model of
perfect orbital and state 'pairing symmetry' for alternant hydrocarbons. 
All-valence NDO procedures like CNDO/S and INDO/S break down this symmetry;
however, the breakdown tends to be too strong, sometimes with unfortunate
consequences for the predicted transitions.  See Spanget-Larsen: Theor. Chem.
Acc. 1997, 98:137-153.  
                             
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JENS SPANGET-LARSEN         Phone:  +45 4674 2000  (RUC)
Department of Chemistry             +45 4674 2710  (direct)
Roskilde University (RUC)   Fax:    +45 4674 3011 
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