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[ESPResSo] Question about Tabulated Interaction

From: Lorenzo Isella
Subject: [ESPResSo] Question about Tabulated Interaction
Date: Tue, 1 Jul 2008 11:22:04 +0200

Dear All,
I would like to run some simulations to investigate the dynamics of
nanoparticles which are supposed to interact via a potential derived
from the integration of a Lennard-Jones potential over two spheres
(each particle is much larger than a molecule).
Now, I have the analytical expression for both the potential and the
force, so I am able to generate my own tabulated interaction. I simply
wonder how I should choose the minimum separation between these two
Namely, the potential blows up becoming infinitely repulsive when the
surface-to-surface distance, d, becomes zero and exhibits a minimum
above that distance before decaying quickly to zero.
So, physically, there is no possibility of an interpenetration of the
two particles, but I do not know how I should treat it numerically. It
seems that I have to specify the tabulated potential as a function of
the centre-of-mass-to-centre-of-mass separation between the two
particles, if I understood correctly.
But strictly speaking this is not defined for a
centre-of-mass-to-centre-of-mass distance smaller than 2r_p, where r_p
is the particle hard-core radius.
I think that a similar problem should arise also in the standard
Lennard-Jones potential treated for particle separations below r_off.
So, how is that dealt with in Espresso?
I am concerned that, during the evolution, a particle may end up in a
region where it cannot possibly have access to and that it would not
feel any repulsion at all.
What about introducing an artificial repulsive force for small separations?
I know this has been done in similar situations, but first I'd like to
know what is the standard treatment of such cases in Espresso.
Many thanks


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