ionsiation potentials/electron affinities

[CTARG-: at :-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