[gmx-users] Scaling Coulomb interactions with lambda, on a pair-wise basis

[Berk Hess]

>From: Matt Wyczalkowski <maw2 at cec.wustl.edu>
>Reply-To: Discussion list for GROMACS users <gmx-users at gromacs.org>
>To: gmx-users at gromacs.org
>Subject: [gmx-users] Scaling Coulomb interactions with lambda,on a 
>pair-wise basis
>Date: Wed, 05 Dec 2007 15:10:57 -0600
>
>Hi,
>
>I am looking to scale non-bonded interactions between two atoms with the 
>lambda parameter, while keeping other interactions unchanged.  I am not 
>sure how to do this for Coulomb interactions.
>
>Scaling the Lennard-Jones interactions between a specific atom pair seems 
>straightforward, by setting VA, WA, VB, WB for that pair in the [PAIRS] 
>directive.  However, in order to modify Coulomb interactions between an 
>atom pair, it seems I need to modify qA and qB for each atom -- this then 
>affects the interactions between this atom and all other atoms as well, 
>something I need to avoid.
>
>Is there a way to scale Coulomb interactions for a specific pair of atoms 
>only?

Unfortunately this can not be done easily.
The only way would be to use user tables for the interactions between
these two atoms and use the dispersion table for LJ and the repulsion
table for Coulomb and the Coulomb table zero. In that way you can use WA/WB
as charge parameters.

It would be useful to have Coulomb scaling for pairs, mainly for forcefield 
issues
where you could have different scaling parameters for different molecules.

But why would you only want to turn of the Coulomb interaction between two 
atoms?
I don't see how that could be of use.

Berk.

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