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Re: [ESPResSo-users] Non neutral charged system

From: Florian Fahrenberger
Subject: Re: [ESPResSo-users] Non neutral charged system
Date: Sat, 10 Mar 2012 19:19:47 +0100
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:10.0.2) Gecko/20120216 Thunderbird/10.0.2

Hi Salvador!

> polymer $num_polymers $num_monomers_per_polymer 1.0 charge 1.0 distance
> 3 types 0 1 bond 0
> counterions [expr $num_polymers*$num_monomers_per_polymer] charge -1.0
> type 2

Yes, this means that only every third monomer carries charge. (actually,
every monomer carries 1/3 charge, but that's a technicality.

> This means that the system has not a
> neutral charge
> However when I run the simulation, ESPResSo does not complain about not
> charge neutrality. So, I was wondering how EXPResSo handles no neutral
> charged systems.

As you very well pointed out, 3D periodic systems should be
charge-neutral, to avoid mathematical difficulties. Many (not all)
algorithms in ESPResSo handle this in the following way: The total
charge of the system is calculated. If it is not zero, the remaining
counter-charge will be smeared as a constant charge density on the
background. This will not influence the potential derivative and
therefore the force, so the simulation is kept intact, but it gets rid
of the mathematical inconsistencies of non-neutral systems.

I hope that helps. Greetings,


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