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Re: [ESPResSo-users] Command galileiTransformParticles

From: Owen Hickey
Subject: Re: [ESPResSo-users] Command galileiTransformParticles
Date: Thu, 15 Dec 2011 12:12:50 +0100


I am not familiar with system_com_vel although I would try:
puts [system_com_vel]

and see what it does.  Presumably it returns a list which you could
store in a variable instead of printing to screen.  For
galileiTransformParticles one simply needs to write that command with
no arguments to adjust the center of mass velocity to zero.  I am not
surprised that the system seems to drift without this using DPD as
numerical errors will eventually give the fluid a net velocity which
the dumbbell will follow.  The rotation seems a bit stranger.  The
reason that this does not happen with the Langevin thermostat is that
it is an implicit fluid with zero velocity (this is the
-friction*velocity term).  If you use both thermostats you should fix
the drifting problem but the Langevin thermostat will also kill
hydrodynamic interactions and thus defeat the purpose of simulating a
DPD fluid.

Hope that helps,


On Thu, Dec 15, 2011 at 2:17 AM, Salvador H-V <address@hidden> wrote:
> Dear All,
> I am trying to simulate a system using DPD thermostat.
> In the user guide i found the following sentence: "... before using dpd, you
> have to stop the center of mass motion of your system, which
> you can achieveby using the command galileiTransformParticles. This may be
> repeated once in a while for long runs due to round off errors (check this
> with the command system_com_vel) "
> I did no find more information about these two commands.
> I would appreciate any help of how to apply both commands inside an ESPResSo
> simulation. What is the syntaxis?
> By the way, using the 2.1.5 version of ESPResSo I observed some unexpected
> behavior for a single dumbbell system using DPD thermostat.  The dumbbell
> seems to translates and rotates only in one direction. When I use the
> Langevin thermostat it seems to perform both translation and rotation
> random/brownian movement.  I using the same system with both thermostats.
> Any comment will be greatly appreciated.
> Thans a lot,
> Salvador H-V
> --
> =o=o=o=o=o=o=o=o=o=o=o=o=o=o=o=o=o=o=o
> Dr. Salvador Herrera Velarde
> Division de Ciencias e Ingenierias
> Campus Leon
> Universidad de Guanajuato
> =o=o=o=o=o=o=o=o=o=o=o=o=o=o=o=o=o=o=o
> Este correo ha sido editado para evitar el uso de acentos y guardar
> compatibilidad entre
> diferentes distribuciones

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