[AMBER] Dielectric constant and scaled charges

From: Lorenzo Gontrani <l.gontrani.caspur.it>
Date: Wed, 18 Apr 2012 12:34:09 +0200

Dear Amber users, I would like to pose you the following question.

I am simulating a charged system (a bulk liquid ionic made up of
cations and anions, but the same problem should apply, in principal,
to any ion-molecule system).

>From a series of ab initio calculation, I obtain that a great amount
of charge transfer takes place between cation and anion (I calculated
the point charges with various methods, such as RESP/HF and CHELPg/MP2
or others, on small models like ionic couple, tetramer and hexamer).
To account for this phenomenon in the classical simulation, without
using a polarizable force field, I am trying to rescale the ab initio
charges (calculated in vacuo for the free ion) by a given percentage
(for instance, 0.9, 0.8 and 0.7). I thought that the same effect could
be obtained by using a dielectric constant in the simulation, e. g.
eps=1.5625 if I scale by 0.8; 1/(0.8*0.8)=1.5625.
But the results I obtain with direct charge scaling and with the
modified dielectric constant are rather different. Am I making a great
mistake, or does the dielectric constant play a role that I don't see
(for instance, in Ewald sums?)

Thanks for any suggestion


 Lorenzo Gontrani
 Research associate of CNR-ISTM (Rome Tor Vergata)
 EDXD group of University of Rome "La Sapienza"
 GSM +39 338 7615798
 Email l DOT gontrani AT caspur DOT it
 Webpage: http://webcaminiti/gontrani.html
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Received on Wed Apr 18 2012 - 04:00:03 PDT
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