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From: Lorenzo Isella
Date: Wed, 07 May 2014 10:28:45 +0200
User-agent: Opera Mail/12.16 (Linux)

I implemented by hand a similar feature (by some modification of the expresso code) some years ago for a paper. I modelled the dynamics of combustion-generated nanoparticles (larger than the air molecules), consisting of several spherules bound together by a short-ranged potential. The Langevin noise was used to model the gas-to-spherule collisions, which is also the mechanism giving rise to the friction. Now, the inner spherules, deep inside the aggregate, are more difficult to reach from the air molecules (hence they are shielded) than those on the surface and as a consequence they experience a different friction.
Have a look at



On Wed, 07 May 2014 09:27:15 +0200, Marcello Sega <address@hidden> wrote:

Hi guys!

For the record: it does indeed make sense to have different frictions.
Think about a simple electrolyte. Anions and cations have different
effective friction coefficients because of two effects, namely 1) the
Stokes friction itself (ions and cations have different sizes) and 2)
the different way they interact with water. This translates to
different specific conductivities of the two type of ions, and in a
coarse-grained simulation setting a separate friction coefficient is a
straightforward way to model this (combined with using different
particle masses).


On Wed, May 7, 2014 at 9:19 AM, Tristan Bereau <address@hidden> wrote:
Dear Olaf and Stefan,

Thanks for the quick replies.

The meaning is indeed very clear--I wasn't pointing at any lack of
documentation. I was merely curious as to whether any scientific study
had used such a scheme. I can actually think of several applications
where this could be of interest.


On Wed, May 7, 2014 at 9:01 AM, Olaf Lenz <address@hidden> wrote:

I admit that the docs are very compact, but on the other hand the meaning is relatively clear, isn't it? However, it would be a good idea to also hint to
the feature in the Langevin description.

I am not aware of any real physical application. We use it mainly for
demonstration purposes, see Stefan's mailing.


2014-05-07 8:55 GMT+02:00 Tristan Bereau <address@hidden>:

Dear all,

I've just stumbled across this wonderful LANGEVIN_PER_PARTICLE feature
in ESPResSo, and was wondering if anyone could point me to some
relevant literature, especially of applications. I couldn't find any
reference from the user manual.

Many thanks.


Dr. rer. nat. Olaf Lenz
Institut für Computerphysik, Allmandring 3, D-70569 Stuttgart
Phone: +49-711-685-63607

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