Re: [ESPResSo] Specification of External Potential/Force

[Torsten Stuehn]

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 
EMail address@hidden

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