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Re: [ESPResSo] Specification of External Potential/Force

From: Torsten Stuehn
Subject: Re: [ESPResSo] Specification of External Potential/Force
Date: Tue, 19 Feb 2008 19:23:09 +0100
User-agent: Thunderbird (X11/20070221)

Hi Lorenzo,

external additional forces that can be applied to particles with the
"part <id> ext_force <x> <y> <z>" command are only constant values
that are added to a particles force-vector e.g. to simulate gravitation.

If I understand you correctly, you are looking for a way of adding a
force to the particles that depends on the z-coordinate of the simulation
You could do this by adding a wall-constraint to your simulation.
The interaction of constraints (like a wall) with particles is like a
non bonded interaction between to particles.
Unfortunately there is currently no harmonic force/potential implemented
for non bonded interactions. You could help yourself with a tabulated
potential (file: 1st line: # <number of values> <minval> <maxval>
following lines: <distance> | <force> | <energy>    ,ESPResSo will interpolate
linearly between values):

 inter 1 0 tab <filename>
constraint wall normal 0 0 1 dist 1 type 1
Do the same for x and y direction.
I never tested a set of rectangular wall constraints though, but it may work.

(2) Yes, if you switch off the thermostat, you will run a
"microcanonical simulation". The total energy of your system will be
a constant. However, if you make many integration steps the total
energy of the system could rise due to numerical errors. So you
should check it from time to time.


Lorenzo Isella wrote:
Dear All,
I think I have a quite unusual problem here.
I am often (ab)using Espresso to investigate systems and problems quite far away from molecular dynamics. Now, I would like to consider a system in 3D where you have a pair-wise interaction between particles (not relevant right now, but you can think of a Lennard-Jones potential) plus an external potential acting on the system. To fix the ideas, think of a strong harmonic confinement along z, i.e. V_z=K_z*z^2 and of a much milder confining potential in the xy plane i.e. V_{xy}=K_{xy}(x^2+y^2), with K_z>>K_{xy}. This is intended to give rise to rather flat structures along along z (together with an interaction potential having a hard-core repulsion). (1) Is this doable in Espresso? Can one tune the external force/interaction in a similar way as one can do with the tabulated interaction potential?
In the documentation I bumped into:

[ext_force x value y value z value] An additional external force is applied to
the particle.

when discussing particle properties, but I am not sure this allows me to do exactly what I have in mind [See page 26 of the user guide]. (2) If I turn off the thermostat, am I using Espresso for deterministic molecular dynamics? Am I then removing the "noise" (apart from numerical round-off errors) from the simulation?

Many thanks


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Dr. Torsten Stühn
MPI für Polymerforschung
Ackermannweg 10
55128 Mainz / Germany

Tel.  +49-(0)6131-379268
Fax +49-(0)6131-379100 EMail address@hidden

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