Subject: Charges

From: "Schiffer, Heinz, Dr., WIA / SC" <[email-masked]>
Date: Thu, 24 Oct 1996 15:14:52 +0200

Hi all,
>from time to time there is a discussion about the construction and the use
of point
charges in molecules. I think there is no doubt within the community, that 
for
intermolecular interactions one should use charges ( and point dipoles, 
point
quadrupoles, etc ) which are adjusted to reproduce as good as possible the
electrostatic potential of the molecule. For qualitative discussions of 
chemical
bonding the use of various population analysis ( Mulliken; 
Davidson-Roby-Ahlrichs,
natural orbital populations of  Weinhold; Bader; etc. By the way : There is
a very promising new one of Marco Haeser in JACS, Vol. 118, 1996, pp. 
7311-7325,
which I think is more rigorous as any previous ones and without any ad hoc
postulates.) are most appropriate.  The fitting of the electrostatic 
potential of a
molecule is a highly non-trivial subject ( see e.g. Michelle Miller Francl 
et al.,
J. Comput. Chem., Vol. 17, 1996, pp. 367-383, and the very recent review by
Sarah L. Price, J. Chem. Soc., Faraday Trans., Vol. 92, 1996, pp. 2997-3008 
)
and in my view not a really solved problem, especially for large molecules.
Now I became aware of a series of papers by George G. Hall and his coworkers
about the fitting of electron densities, which I never heart off bevor and 
which, I think,
are quite unknown to the community. Hall et al. fit the calculated electron 
densities
of molecules to atom-centered gaussians as it is done by many DFT-programs
( "density fitting with an auxillary basis" ) and got a very good ( in my 
view an
excellent ) representation of the electrostatic potential of a molecule. 
Those DFT-
programs with charge density fitting are able to calculate the charge 
density of
molecules  within density functional theory with more than thousand basis 
functions
on a single workstation in about an hour of cpu-time ( see e.g. Karin 
Eichkorn et al.,
Chem. Phys. Lett. 240 (1995) 283-290 ). As a by-product of the calculation 
one gets
an excellent fit of the electrostatic potential of the molecule with 
gaussians. After some
minor modifications of the programs one gets from this point charges, point 
dipoles,
point quadrupoles, etc. or if one likes it the current bun model of Hall and 
coworkes.
My question to the community is now, wether anyone else has similar thoughts 
or
even experiences with this sort of density fitted electrostatic potentials ?
Does anyone has an opinion about using such charge models in force field
calculations ( energy minimization of molecular crystals or molecular 
dynamics
calculation of fluids ) ? Please send responses to the list, so that anyone 
else can
follow the discussion.
Ciao,
Heinz

PS: Here are the references of Hall and coworkers :

     George G. Hall and David Martin
     Approximate Electron Densities for Atoms and Molecules
     Isr. J. Chem. 19 (1980) 255-259

     G. G. Hall and C. M. Smith
     Fitting Eelectron Densities of Molecules
     Int. J. Quantum Chem. 25 (1984) 881-890

     G. G. Hall and K. Tsujinaga
     The molecular electrostatic potential of some simple molecules
     Theor. Chim. Acta 69 (1986) 425-436

     K. Tsujinaga and G. G. Hall
     The currant bun model of simple molecules
     Theor. Chim. Acta 70 (1986) 257-264

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Dr. Heinz Schiffer            65926 Frankfurt am Main
Hoechst AG               Phone ++49-69-305-2330
Scientific Computing          Fax   ++49-69-305-81162

Email :   [email-masked]
     [email-masked]
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From: Konrad Hinsen <[email-masked]>
Date: Thu, 24 Oct 96 14:36:36 +0100

> and in my view not a really solved problem, especially for large molecules.

I don't agree. The charge fitting problem is not essentially different
>from other fitting problems in all branches of science and
engineering, and lots of articles and books have been written about
it. There are well established and robust techniques to find optimal
charges and to judge to what degree they are well determined by the
reference data. Much of this is not generally known in the computational
chemistry community, but that's another problem.

There remains of course the problem that the electrostatic potential
in a reasonable region may not be sufficient to determine all charges
uniquely. But that is not really fatal: if after adding all reference
data you can reasonably get (e.g. from multiple conigurations) some
charges still remain illdetermined, then they probably don't matter
much in real life. If they do, then probably the fixed point charge model
is not good enough.

> excellent ) representation of the electrostatic potential of a molecule. 

Essentially that means choosing a more complicated model (gaussian densities
rather than point charges), and it is not surprising that you can get
better results. The question is whether people are willing to work with
such more complicated models in their force fields. It is certainly
worthwhile exploring in which situations such models can give better
results.
-- 
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Konrad Hinsen                          | E-Mail: [email-masked]
Laboratoire de Dynamique Moleculaire   | Tel.: +33-76.88.99.28
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From: A J Turner <[email-masked]>
Date: Thu, 24 Oct 1996 15:10:02 +0100 (BST)

Hi!

I am working on the charge fitting area right now.  My experiance may be
of interest.  The approxiation of placing charges on nuclear centres can
greatly outweigh to variation in method of attaining the charges in the
first place.

I am using QM/MM technology to _directly_ fit a MM force field to a QM/MM
force field.  This shows that several 'off centre' points _must_
be included if one is to get anything like a reasonable fit of the mm
forces to the qm ones.  My present example being ch3o- , where the forcing
of the negative charg onto the 'centre' of the oxygen causes maiisve
'artificial' +ve charge build up on the carbon and hence large negaive
charges on the protons.  The use of a QM/MM force field where the same vdw
paramters apply shows that these negative protons are very very far from
correct.

Does anyon else have similar experiances?

Best wishes

Alex

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|Alexander J Turner         [email-masked]                  |
|Post Graduate              |http://www.bath.ac.uk/~chpajt/home.html|
|School of Chemistry        |+144 1225 8262826 ext 5137             |
|University of Bath         |                                       |
|Bath, Avon, U.K.           |Field: QM/MM modeling                  |
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