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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 1.5.0.10 (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
box:
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.
Torsten.
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
Lorenzo
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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
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