Subject: RE: 2:electrostatics and dipole moment

From: Joao Brandao <[email-masked]>
Date: Fri, 29 Mar 1996 17:09:25 +0100

The dipole moment is just one term in a multipolar expansion of the 
electric  charge density of the molecules. All this expansion must be done 
with the same center. If it is the first non-vanishing term, its value is 
origin-independent, and we can use what origin we want, BUT the remaining 
terms in the multipolar expansion change their values with our choice for 
this origin. 
    The dipole-charge energy will have a different value, BUT ONLY the 
complete expansion will converge to the same energy whatever the origin we 
consider. That means that the quadrupolar term may became important. The 
same problem arrives if we compute charge-charge energies. 
    The center of mass or the center of charge are the most common used 
centers.

Joao Brandao
---------------------------------------------------------------------
Unidade de Ciencias Exactas e Humanas
Universidade do Algarve -Campus de Gambelas
P-8000 FARO
Portugal
e-mail: [email-masked]
Tel. 0351-089-800914 ext. 7628
Fax. 0351-089-818560
---------------------------------------------------------------------

----------- End Forwarded Message -----------

---

From: [email-masked] (Konrad HINSEN)
Date: Fri, 29 Mar 1996 14:21:26 -0500

>     The center of mass or the center of charge are the most common used 

The center of charge will give the fastest convergence of the
multipole expansion. To get the largest region of convergence, choose
the origin to minimize the largest distance of any charge (keep in
mind that a multipole expansion converges only inside a sphere
containing all the charges). The center of mass has of course
nothing to do with charges or multipoles, but it is often a
convenient choice for other reasons.

-------------------------------------------------------------------------------
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. Centre-Ville     | Deutsch/Esperanto/English/Nederlands/
Montreal (QC) H3C 3J7             | Francais (phase experimentale)
-------------------------------------------------------------------------------