Subject: Re: CCL:Question about Electrostatics in Molecular Mechanics

From: [email-masked] (Per-Ola Norrby)
Date: Fri, 4 Aug 1995 09:52:43 +0200

At 16.34 95-08-03, BILL WELSH wrote:
>Dear Netters,
>
>Please help settle a small controversy.  For a particular molecular
>mechanics calculation, I derived partial atomic charges from bond
>dipole moment measurements made in nonpolar solvent.  I then ran
>molecular mechanics calculations using an in vacuo dielectric constant
>of 1.00.  A colleague argues that I must use a dielectric constant
>appropriate to the nonpolar solvent from which the charges were
>derived (e.g., 2-3).  It seems to me that the charges already embody
>the effect of the nonpolar solvent's dielectric constant, and so using
>the in vacuo value of 1.00 is okay and perhaps preferred.  Stated another
>way, it seems to me that using these charges AND the dielectric of 2-3
>would count the effect of the nonpolar solvent twice.
>
        Dear Bill

        How to handle charges correctly is probably one of the hardest
problems in molecular mechanics today.  The recommendations usually vary
between the force fields too, so some details on your system would help.
However, almost all mature methods use environmentally independent partial
charges (or bond dipoles, ...).  The parameters of your force field are
probably derived to reproduce gas phase data, so you should expect best
results if you also want to predict gas phase or in vacuo behavior.  If you
want to predict how the molecules behave in a solvent, you should add a
solvent model.  The simplest possible solvent model is changing the
dielectric constant.

        So, what are you trying to predict with your calculations?  What
force field are you using?

        Best Regards,

        PeO Norrby

 *  Per-Ola Norrby
 *  The Royal Danish School of Pharmacy, Dept. of Med. Chem.
 *  Universitetsparken 2, DK 2100 Copenhagen, Denmark
 *  tel. +45-35376777-506, +45-35370850    fax +45-35372209
 *  Internet: [email-masked], [email-masked]

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From: [email-masked] (Tony Schmidt)
Date: Fri, 4 Aug 95 08:57:38 -0400

On Aug.8 Bill Welsh wrote:

Dear Netters,

Any opinions on this matter are appreciated.

Thanks ...

Bill Welsh
Dept. of Chemistry
Univ. of Missouri-St. Louis
-----------------------------------------------------

If you have dipole moments in solvent, you may have to scale them for vacuum
using e.g. strategy by Sharp et al, J.Phys.Chem. 96, 3822 (1992).

Tony Schmidt
[email-masked]