From CTARG@Levels.UniSA.Edu.Au  Sat Oct  9 01:25:44 1993
Date: Fri, 08 Oct 1993 15:55:44 +0930
From: CTARG@Levels.UniSA.Edu.Au
Subject: ionsiation potentials/electron affinities
To: chemistry@ccl.net
Message-Id: <01H3VIZGR9YQ9LVEEL@Levels.UniSA.Edu.Au>


Hi Netters,
           The question I have to ask probably falls into the
category of simplistic rubbish that I should look up in a 
school library somewhere but never mind....

I have been trying to reproduce the Gasteiger charge coefficients
as used by Chem-X with the view to parameterising some other atoms.

The reference paper 'Iterative partial equilization of orbital
electronegativity - a rapid access to atomic charges' by Gasteiger
and Marsili, Tetrahedron, vol36,(1980) pp 3219 to 3288 gives
the a parameter of the electronegativity polynomial as
 
     a=1/2(I+E) where I is the ionisation potential and E is the

electron affinity. For Hydrogen the text value for a is given as
7.17 eV which is fine using literature values of I and E. However
I can't seem to reproduce any of the polynomial parameters for
any other atoms. The type of bonding to each atom is obviously taken
into account as different a, b and c coefficients are quoted for
different bonding arrangements. How is ionsiation potential of an ATOM
calculated/measured when it is bonded to something else? 

Can somebody enlighten me as to how these values are arrived at?
Sorry about being so simple minded but somebody has to ask theses
questions to make everybody else feel good!

        Thanks
           Andy