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## Re: [ESPResSo-users] Coulomb forces capping

 From: Peter Košovan Subject: Re: [ESPResSo-users] Coulomb forces capping Date: Fri, 15 Aug 2014 17:45:09 +0200

Hi Aleksei,

if overlaps cause your trouble, let me ask how you make your spheres big with the LJ potential? Do you vary sigma, or add the offset? If you vary sigma, then your spheres can normally approach each other to ~0.9 sigma (assuming epsilon=1kT). If you are seeking to model quite hard and big colloidal particles, I would suggest to control the sphere size by the LJ offset, keeping sigma=1.0. Then the spheres should never approach closer than offset+0.9sigma, which could resolve your problem with overlaps.

Note, that if you add offset, you have to set up your system such that there are no initial overlaps. Otherwise, Espresso complains. A possible solution is to start with zero offset and electrostatics OFF, gradually increase offset to the desired value, and then switch on the electrostatics.

Regards,

peter

2014-08-15 16:27 GMT+02:00 Axel Arnold :
Hi!

Capping the Coulomb forces is not possible, since part of them is long-range, where capping is mathematically extremely difficult. The same holds for shifting the Coulomb interactions.

We could cap the short-range forces, but then the strength of capping depends on the tuning of P3M. Such a capping can only be used for warm up.

It might therefore be better to just set up the system from the very beginning that no overlaps occur, and then equilibrate with full interactions.

Best,
Axel

On 15.08.14 16:14, Aleksei Kabedev wrote:
Dear all,

I'm not sure that's the best way to do it, but I'm modeling some big spheres by just central points, giving them LJ potentials and some charges. The problem is that if the radiuses are too big, I have to give really big charges to the particles to keep the right surface charge density, as the potential decreases up to growth of the radius exponentially. That wouldn't be bad if some overlaps didn't appear from time to time. In that case even a small intersection causes huge forces and velocities.
As far as I understood there is no forcecapping for the electrostatic interaction in espresso. Is there any chance to implement it easily? Or probably there is a way of shifting the coulomb forces action range?
Thank you!

--

Kind regards,
Aleksei

--
JP Dr. Axel Arnold
ICP, Universität Stuttgart
Allmandring 3
70569 Stuttgart, Germany
Tel: +49 711 685 67609

--
Dr. Peter Košovan

Departmtent of Physical and Macromolecular Chemistry
Faculty of Science, Charles University in Prague, Czech Republic

Katedra fyzikální a makromolekulární chemie
Přírodovědecká fakulta Univerzity Karlovy v Praze

www.natur.cuni.cz/chemistry/fyzchem/
Tel. +420221951290
Fax +420224919752

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