From: Philippe Hiberty <[email-masked]>
Date: Tue, 25 Apr 1995 14:25:25 +0100

Dear Dr. Collins
	The way to study a heterolytic dissociation process is to use
ab initio valence bond theory. You can define very clearly a specific
valence bond structure, such as F-H+, and follow its energy along
a reaction pathway (a dissociation in your case). For this to be
meaningful, the valence bond method must use orbitals strictly 
localized on either fragments. You need also a VB method that optimizes
the orbitals. 
	An applicatory example of such a method is displayed in J. Chem.
Phys. 101, 5969 (1994). In this case we have studied a homolytic dissociation
but to study the heterolytic one would be just as simple.
	Now in the particular case of FH, you don't even need a VB program
to calculate the F-H+ structure at any interatomic distances. You can use
any standard ab initio methodto do that. Suffices to define a funny basis
set for the departing hydrogen, i.e. a single orbital either extremely
diffuse or extremely contracted (to my experience, this works well with
a very diffuse basis function). This way no electron goes in such an orbital,
and the wavefunction you calculate just corresponds to F-H+. By the way
it is easy to check by looking at the occupied MO's that noone of them
has any component on the departing hydrogen.
	Hoping these comments will be useful,
               Sincerely yours,
    P.C. Hiberty

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