Subject: Re: CCL:Ewald Sums and Amber 1-4 interactions.

From: Konrad Hinsen <hinsen:at:cnrs-orleans.fr>
Date: Wed, 1 Oct 2003 09:24:17 +0200

On Tuesday 16 September 2003 20:42, you wrote:

> Amber scales the 1-4 electrostatic terms by a scale factor.  It's unclear
> to me how this should be accounted for in the Ewald method.  These

By doing a standard Ewald sum (no scaling whatsoever) and then subtracting 
what was counted too much, i.e. subtracting a few standard Coulomb terms 
calculated in minimum-image convention.

> interactions are already included in the k-space sum and usually are
> corrected for in the real-space exclusion correction.  Its not clear to me
> how the scaled-1-4 term can be accounted for other than to do the
> real-space Ewald treatment of the 1-4 term and then multiply in the Amber
> scaling term.

If you do that, then you have counted the 1-4 term fully in the k-space term 
but scaled in the real-space term. This is inconsistent, in particular it 
will make your energies dependent on the Ewald screening parameter.
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