From:	"J. Fleischhauer" <JF { *at * } ctch20.itc.tu-clausthal.de>
Date:	Sun, 17 Dec 1995 09:39:32 GMT+1
Subject:	charge equilibration

Dear Netters


In our approach to describe charge influences in Force Field
calculations, we want to try the an empirical method so called
 (QEq), described in :
A. K. Rappe, W. A. Goddard, J. Phys. Chem., 95, 3358, 1991.

This method is also available in the commercial CERIUS2 package.

Our problem is that there is no detailed description of the
algorithm.
Does anybody knows other sources of information dealing with the
charge equilibration algorithm?

Especially we can't find an mathematical expression used for the
calculation of the coulomb potential between center A and B = Jab(R).

The Problem is the following :
The simple coulomb expression between unit charges on center A and B
separated by the distance R is :

(14)                     Jab(R) = 14.4 / R

14.4    conversion factor, R in Angstroem, Jab in eV

Equation (14) is no longer valid for atoms with overlapping charge
distributions (e.g. mostly bonded atoms). For R = 0 (14) leads to an
infinite value whereas Jab(R=0) should reach a finite value Jaa
instead. Like described in the reference, they use a shielding
corrections to (14). The shielding to (14) will be quite large for
bonded atoms (small R). The shielding correction to (14) were
described in terms of a coulomb integral between atomic densities
witch are expressed in form of slater orbitals.
(eq.15)


(15)        Phi_slater = Nn * r power( n-1 ) * exp(-zeta*r)

Nn      normalisation constant
n       long periodic row
zeta    Slater exponent
r       distance


The diatomic Coulomb integral Jab is evaluated exactly.


Our question :

1) Witch is the correct mathematical expression to describe the
   coulomb interaction Jab(R) to get a potential with a finite
   value for R=0 ?







_________________________________________________

Dipl. Chem. Juergen Fleischhauer

Technical University Clausthal
Dept. of Technical Chemistry
Erzstr. 18
D-38678 Clausthal Zellerfeld
Germany

Tel.    : 05323-72-2647
Fax.    : 05323-72-3655
E-Mail  : jf -8 at 8- ctch20.itc.tu-clausthal.de
__________________________________________________


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