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Re: [ESPResSo-users] Question of NVT ensemble

From: Florian Weik
Subject: Re: [ESPResSo-users] Question of NVT ensemble
Date: Mon, 3 Nov 2014 11:39:56 +0100

Hello Joe,
as Xikai Jiang wrote ESPResSo does not implement velocity rescaling thermostats (one of the reasons for that is that there are well known difficulties with this approach, if you don't now about that you should google for "Flying ice cube" effect.). The default thermostat in ESPResSo is a Langevin thermostat. DPD can also be used, but this is not the default approach and should only be used if the other features of DPD are needed. Langevin dynamics introduces a friction term into the equations of motion for the particles which is proportional to the velocity of the particle. The translational velocity you get is the one at which the electric force on your chain and the friction force on its particles are in balance. As such the stationary velocity you get depends on the friction you choose. This somewhat models the interaction between molecules and the solvent which in typical physical situations is present in reality. The DPD thermostat can be configured in such a way that it only has friction on the particle-particle interactions, so that the center of mass movement hast no dampening on it. In this case the translational velocity will diverge with time, because the external field keeps pumping energy into the system but no energy is dissipated.
Typical (coarse grained) simulations of charged polymers use the Langevin thermostat, or something more elaborate like a Lattice-Boltzmann fluid if hydrodynamic interactions should be explicitly modeled.
The choice of the thermostat is not a merely technical one, but should correspond to the physics of you system. Please read the relevant sections of the ESPResSo User's guide before you start simulations. If you want further help, you should describe more closely to what end you want to do your simulation and what physics you want to capture.


On Thu, Oct 30, 2014 at 6:00 PM, Joe <address@hidden> wrote:
Hi Guys,

If I would like to simulate a charged chain move under the external electric field and using NVT ensemble, will it cause a reduction of the translational velocity? From the text book I know that the way NVT works is keep rescaling the velocities to keep the temperature constant. But in this scenario, will the drift velocity also be rescaled?

How to avoid this problem?


Florian Weik, Dipl.-Phys.,
Institut für Computerphysik, Allmandring 3, D-70569 Stuttgart
Phone: +49-711-685-67703
Public Key 0x0562F11D Fingerprint 3294 6360 EC93 37A3 BD40  F597 0BAD 3AF8 0562 F11D

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