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Re: [ESPResSo] Espresso: Surface Force Analysis

From: Axel Arnold
Subject: Re: [ESPResSo] Espresso: Surface Force Analysis
Date: Mon, 7 Jul 2008 09:37:38 +0200
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> The stress tensor approach might not be the right approach if Espresso has
> a way to displaying outputs for the force fields, or forces exerted upon
> an atom. We are not sure about a command from the User Guide, but this
> could probably be done by looking deeper at the algorithms.


if you want to see the total forces acting on a particle, you can just use 
"part $id print force" in Tcl; however, doing this every time step is 
extremely inefficient.

Another thing to consider is that the total force on a particle includes the 
random forces from the thermostat, if you plan to use one. The typical 
approach if you want to measure forces is to do so every couple of 100 time 
steps to improve statistical independence, then switch off the thermostat, 
calculate the forces, and then continue, i.e.
"thermostat off; integrate 0; part $id print forces; thermostat langevin 1 1"
In case you are interested in the forces only from a specific interaction, you 
will need to temporarily switch off the other interactions, by setting the 
prefactor to 0 (e.g., k for FENE, eps for Lennard-Jones).


Dr. Axel Arnold 
Fraunhofer SCAI
Schloss Birlinghoven, 53754 Sankt Augustin, Germany
Tel: +49 2241 14 2575

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