Subject: BSSE and chemical ambiguity (fwd)

From: Patrick Bultinck <[email-masked]>
Date: Wed, 29 Sep 1999 16:55:52 +0200 (DFT)

Dear,

I have a problem with the following. Say, a quantum chemist wants to 
calculate the thermodynamic quantities for the following reaction :

A-B	->	A + B

He needs a couple of energies, but also the vibration frequencies. So he 
goes off to calculate the hessians of all fragments and the complex. A 
and B should be in a minimum to calculate Delta H (e.g.). The interaction 
energy is calculated in CP way. But now... He wants to optimize A 
and B starting from their geometries as they are present in the complex.

Option 1 : He optimizes A with A monomer basis set, and similar for B.
As such he obtains 'true minima' for A and B, chemically relevant, but 
looses BSSE correction in Delta-H on the way.

Option 2 : He optimizes A and B both separately, but in the supermolecule 
basis set. BSSE can be corrected for. BUT... who knows in what way the 
presence of ghost orbitals acts in the optimization ? And what is Delta-H 
worth chemically for A and B in a minimum that gives rise to chemical 
question marks ? It is clear that doing optimizations with the 
supermolecule set, yields some chemical and mathematical ambiguities 
(also noted and published by Mayer et al.).

The matter is getting worse. Consider A a ligand with 50 minima. We need 
to get the relaxation of A to the global minimum, since that is most 
occupied, and considering relaxation to say the 41th minimum (the 
minimum that might resemble the distorted geometry of A best) bypasses 
some distorion energy and distorsion delta-H. But how do I get to the 
global minimum from my distorted A geometry when I use the supermolecule 
set. I could twist a couple of dihedral maybe, but how to twist the ghost 
orbitals along ;-) ?

Am I too bold at saying that the best option is to go for the biggest 
basis set you can afford, and try to keep BSSE as small as possible (you 
might get an idea from the interaction energy BSSE correction) ? I have 
tested cases in which I used both options for small A and B, and found 
very little difference. But I would like to know how other people would 
tackle the problem when A and/or B are big, and exhibit a lot of minima. 
My opinion is to include BSSE in the calculation of the interaction 
energy, consider distrotion energies in the monomer basis sets (if 
necessary no problem to calculate using the global minimum as a 
reference), and calculate thermodynamic quantities also in monomer sets. 
I admit that my results will have a certain error flag due to not 
inclusing BSSE corrections, but I think it is chemically still intuitive 
and correct, and errors originate from an incomplete basis set.
The other option corrects for BSSE but bends away (too much ?) from 
straight chemical relevance and intuition. After all we do not optimize 
H2O with sodium orbitals 'somewhere' in the neighbourhood if we want to 
get good vibrational frequencies for free H2O e.g. I agree that the more 
complete the set the better, but I think that does not allow the use of 
imbalanced basis sets.

Any ideas,

Patrick Bultinck
Quantum Chemistry Group
University of Ghent
Belgium