Subject: RE: Dipole moment origin and electrostatic

From: Artem Masunov <[email-masked]>
Date: Tue, 26 Mar 1996 19:41:46 -0500 (EST)

Patrick Bultinck <[email-masked]> wrote:

>All of the ion-ligand interaction is moved into the ion-dipole
>interaction, what about the rest of the multipoles ? This is especially
>interesting since when repeating some calculations by him, I found that  
>he simply put the origin of the dipole in the center of mass. I would
>think that if you want to reduce the ion-ligand interaction to an
>ion-dipole model, you should choose your origin such that most of the
>higher order poles are minimal, so that you make the smallest mistake.

   Nothing surprising. Why bother choosing origin if center of mass gives
you the value good enough? Most of the electrostatic calculations are
driven by wishful thinking. That is why Kitaura-Morokuma decomposition
analysis is so popular: it almost always points to electrostatic. When it
tells you H3B-NH3 complex is strong electrostatic in nature (it does 
indeed), do you believe that too? Did you try NBO analysis (it usually 
points to charge transpher) or geminal analysis (which stresses 
polarization)? Any multipole decomposition is giong to add even more 
confusion.
   What I would do is numerical integration of the charge density to get
rid of all this multipolar approximations. It even takes less effort to do
this. In Gaussian94 you just use GRID to represent electron density as the
set of point charges and then CHARGE to calculate interaction of these
charges with anything else. 

Artem

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