Subject: Re: CCL:charges

From: Ryszard Czerminski X 217 <[email-masked]>
Date: Fri, 30 Sep 1994 12:10:58 -0400

First many thanks to Wendy D. Cornell for an excelent summary.

The treatment of INTERmolecular interactions by perturbation theory
has quite a long history (a lot of reference articles and books).
I do not have references at hand but just to give
you few names to look for (in no particular order):
W. Kolos, B. Jeziorski, L. Piela, P. Claverie, A.D. Buckingham,
A. Stone, P. Hobza, M.M. Szczesniak ...

Molecular multipole expansion (as opposed to multicenter multipole
expansion - atom centered charges are the simplest model of this kind)
works properly only in cases when interacting molecules
are smaller (and preferably much smaller) then distance between them.
For two interacting nucleic acid bases it is a disaster.

For calculating electrostatic interactions I have personally used 
approach developed by Claverie and co-workers

-> J. Langlet, P. Claverie et al. Int. J. Quantum Chem., 19 (1981) 299

as well as by Sokalski

-> W.A.Sokalski et al. J. Phys. Chem., 87 (1983) 2803

The main idea of this approach (as it was already mentioned earlier) is
to go beyond charges (and atoms in Claverie's case) in Mulliken scheme. 

This is definitely better description of electrostatics then any atom
based point charges could provide.

The main problem with applying it to large molecular systems

(definition of what is large depends which generation of computers
are we using and/or how many digits of accuracy are we hoping to achieve)

is greater computational complexity (if we include dipoles,
quadrupoles etc...) and/or larger number of point charges not longer
centered on atoms - which results in much higher CPU costs.

Conformational dependence problems would be probably similar as with
point charges derived e.g. from electrostatic potential fitting -
although I am not aware of any detail study on this subject.

>From bio- simulations point of view (~ tousands of atoms)
we are stack with atomic charges models for a while - probably.

One interesting avenue could be to see if there is a way of deriving
point charges equivalent to ESP/RESP fitted charges directly from wave
function i.e. basically generalizing Thole & van Duijnen approach
to higher moments
(- Thole & van Duijnen, "A general population analysis preserving
   the dipole moment", Theor. Chim. Acta 63, 209 (1983).)

This is probably not possible exactly but maybe some further
progress is possible in this direction.

Sincerely,

Ryszard Czerminski
+------------------------------------------------------------------+
| Molecular Simulations Incorporated | phone : (617)229-8875 x 217 |
| 16 New England Executive Park,     | fax   : (617)229-9899       |
| Burlington, MA 01803-5297          | e-mail: [email-masked]     |
+------------------------------------------------------------------+

---

From: [email-masked] (Hinsen Konrad)
Date: Fri, 30 Sep 94 17:09:46 -0400

Actually I wouldn't call multipole expansions perturbation theory.
There is nowhere any assumption about some parameter being small.
It is simply an expansion in terms of the relation between
the radius of the charged system and the distance to the point
where the potential is to be calculated.

This is not exactly true - what you have mentioned is the
condition for fast convergence. You can even treat two
touching spheres with a multipole expansion, but you might
need fifty terms for that. Whether or not that is feasible
depends on the application.

-------------------------------------------------------------------------------
Konrad Hinsen                     | E-Mail: [email-masked]
Departement de Chimie             | Tel.: +1-514-343-6111 ext. 3953
Universite de Montreal            | Fax:  +1-514-343-7586
C.P. 6128, succ. A                | Deutsch/Esperanto/English/Nederlands/
Montreal (QC) H3C 3J7             | Francais (phase experimentale)
-------------------------------------------------------------------------------