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

Berk Hess hessb at mpip-mainz.mpg.de
Thu Dec 6 16:55:57 CET 2007

Matt Wyczalkowski wrote:

> Berk Hess wrote:
>>> 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.
> (This discussion seems more pertinent to the developers list.)
> The idea is to interpolate smoothly between bonded (lambda=0) and 
> non-bonded (lambda=1) interactions between a particular pair of 
> atoms.  For instance, at l=0 the atoms experience a harmonic potential 
> with coulomb and LJ interactions turned off; as lambda increases the 
> LJ and coulomb interactions scale up while the harmonic potential 
> relaxes, so that at l=1 regular non-bond interactions are recovered.  
> This would be useful for free energy calculations, for example.
> The downside of using tabulated energy functions (a different table 
> per lambda value) is that the idea of dU/d[lambda] is lost.

This is not what I meant.
You use one table for all lambda values.
The only reason for using tables is to decouple the choice of charges 
for the particular pair
from all the other charges.


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