Subject: charge equilibration

From: "J. Fleischhauer" <[email-masked]>
Date: Sun, 17 Dec 1995 09:39:32 GMT+1

Dear Netters

In our approach to describe charge influences in Force Field 
calculations, we want to try the an empirical method so called 
<charge equilibration> (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Equation (14) is no longer valid for atoms with overlapping charge 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  : [email-masked]
__________________________________________________