Subject: Reaction Enthalpy and BSSE

From: Patrick Bultinck <[email-masked]>
Date: Sun, 21 Feb 1999 15:38:13 +0100

Dear CCL community,

Consider a complexation reaction M + B -> MB (m is a metal ion, B a lewis 
base e.g.)

I want to calculate the reaction enthalpies, so I need H for M, B and MB. 
This means I also need the vibrational frequencies. These should be 
calculated for M, B and MB, all in their minimum energy geometry. The 
geometry of fragment B as present in MB is usually not a minimum energy 
geometry. Calculation H for MB and M is no problem. However for B has to be 
studied in it's minimum. Now comes what I am wondering is the way it is 
done to avoid following TWO problems :
a) I can take BSSE into account, but that would mean I need to consider B 
in it's supermolecule minimum. Optimizing B towards  minimum using the 
supermolecule set is however very ambiguous (who would do conformational 
analysis of a free ligand with a bunch of ghost orbitals all over the place 
?). As pointed out by Mayer et al. (I can send the reference, but don't 
have it readily available here) studying a supermolecule approach minimum 
for a monomer yields chemical and mathematical ambiguities.
b) I calculate the frequencies for MB in the superset, and for M and B both 
in their monomer sets. This means that I look at M and B in their true 
minima, not some minimum in a supermolecule basisset (where the presence of 
ghost orbitals can influence the geometry of B in a superset in some 
unpredictable way). However calculating Delta-H as DH=H(MB)-H(M)-H(B) would 
then cause neglect of BSSE.

Note : I am talking BSSE correcting in counterpoise way.

I have seen a LOT of articles calculating reaction enthalpies, and it seems 
most would optimize B in the supermolecule set, but that bothers me more 
than a bit since I am used to conformational analyses without putting ghost 
orbitals somewhere (not knowing how they influence molecular geometries e.g.).

Can anybody share an opinion, I will gladly summarize.

Patrick Bultinck
Universiteit Gent-Belgium
Quantum and Coordination chemistry group