Subject: PPP METHOD SUMMARY

From: elewars <[email-masked]>
Date: Wed, 23 Feb 2000 14:20:15 -0500

Wed 2000 Feb 23
SUMMARY OF STATUS OF PPP METHOD

Hello,
here are the answers to my question about the status of the
Pariser-Parr-Pople method of doing calculations on pi systems.
Thanks very much to all who answered.

   EL
-------------
The Question:

Subject:
CCL:PPP Method: status?
 Date:   Wed, 16 Feb 2000 19:04:11 -0500
 From: elewars <[email-masked]>

2000 Feb 16

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?
2) was it used for calculation of UV spectra *at a given geometry* only,

or could it perform realistic geometry optimizations on pi-systems?
3) what has it evolved into: what currently popular method or program
for calculating electronic spectra is an offspring of the PPP method?

Thank you.

E. Lewars
=============
The Answers (#1--#6)
#1

     Wed, 16 Feb 2000 21:06:31 +0000
      From:   [email-masked]

 PPP theory is still used.  It can be quite accurate, and very fast,
when
parameterised carefully.  It is best used for planar molecules in
general.  If
nonplanar configurations are considered one must take care to avoid
twists that
bring components of localised lone pairs out of the plane.

It works best at a frozen geometry.

It can be used to optimize bond lengths by calibrating the expression
    R = a + b*P
where P is the computed bond order for each nearest-neighbor bond type
at
experimental or high quality ab initio geometries.

PPP theory is just the pi part of INDO/S and similar methodologies, and
assumes a
frozen sigma field that is incorporated into the one center parameters.

Because INDO/S and related all valence electron methodologies treat
sigma electrons
explicitly it is superior to PPP theory for pnon-planar molecules.
Geometry optimization for INDO/S spectroscopic methods are not very well
developed
because parameterizations for spectra give poor geometries, in general.

John McKelvey
=====
#2
     Date:     Wed, 16 Feb 2000 19:58:09 -0500
     From:     "David A. Pensak" <[email-masked]>"David A.
Pensak" <[email-masked]>

The best source of information about this is none other than Rudy
Pariser.   He is [email-masked].   He lives here in Delaware having
retired from DuPont a few years ago.   I'm sure he would be most
happy to answer your questions.
=========
#3
      Date:     Thu, 17 Feb 2000 09:49:15 +0100
      From:      Stefan Grimme <[email-masked]>

Hello,
I have written a PPP program which does
PPP/SCI and PPP/RPA calculations for UV and
CD spectra. I'm using it for large supramolecular
structures. It automatically discards all non-pi
atoms from a cartesian input file for the whole molecule and
assigns the parameters. Non-planarity effects and
through-space interactions are treated explicitely.
Its a development version but you can have it if you want.

Stefan Grimme

___________________________________________
       Prof. Dr. Stefan Grimme
    Organisch-Chemisches Institut
     (Abt. Theoretische Chemie)
  Westfaelische Wilhelms-Universitaet
         Corrensstrasse 40
         D-48149 Muenster
     Tel (+49)-251-83 36512/33241
     Fax (+49)-251-83 36515
     Email:[email-masked]
___________________________________________
=========
#4
   Date:    Thu, 17 Feb 2000 11:15:49 +0000
   From:    "Charlton, Michael" <[email-masked]>

Pi-System from Rudolf Naef performs PPP calcs to predict UV/Vis spectra.

Michael Charlton.
==========

#5
   Date:     Thu, 17 Feb 2000 07:51:27 -0500
   From:    "Risser, Steven M" <[email-masked]>

Hi,

As far as PPP goes, it is still used by several groups doing polymer
physics.  The primary solution method now involves full-CI like
calculations, looking at optical excitations.  PPP is NOT useful for
geometries, since you are completely neglecting the sigma bonded
electrons.
There had been work 10-15 years ago on representing the sigma bonds by
harmonic oscillators, and looking at distortions due to the p bonding,
but
that was not too successful.  As far as available programs, I do not
know of
any that I would say is a direct descendant of PPP.  Most of the
semi-empirical methods come from the NDDO school, which is somewhat
distinct.  The other related work is in physics, where a lot of
electronic
structure is done using the Hubbard model (only nearest-neighbor
electronic
interactions).

Hope this helps
===============
#6
Date:     Thu, 17 Feb 2000 14:25:30 +0100
    From:  Jens Spanget-Larsen <[email-masked]>
 Organization: Roskilde Universitetscenter

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.

> 3) 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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Department of Chemistry             +45 4674 2710  (direct)
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