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Re: [ESPResSo] Position-dependent Langevoin thermostat

From: Christoph Junghans
Subject: Re: [ESPResSo] Position-dependent Langevoin thermostat
Date: Wed, 17 Sep 2008 15:04:57 +0200
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Hi Lorenzo & Olaf,

there is no problem in modifying the langevin thermostat as long as you keep the ration between noise and friction. In Espresso the noise and friction prefactors are saved in langevin_pref1 and langevin_pref2.
In theory you just have to change them.

From the implementation side this is more complicated, because in Espresso there is only one global friction constant for all particles. So inside of the function friction_thermo_langevin, you need to know the number of neighbors, the position of the aggregate or whatever.



Olaf Lenz schrieb:
Hash: SHA1

Hi Lorenzo!

Lorenzo Isella wrote:
There is a fundamental problem with this approach: once a monomer is
surrounded by other monomers, it should feel a different noise due to
the screening by its neighbors, but this effect is not accounted for
in Espresso. I refer to the case in which there is no shear, just
Brownian motion.
Nobody knows exactly what it should feel, but it would be (at least
for me) wonderful to have a feature in Espresso that allows me, while
keeping the overall system temperature around T, to have the noise on each
monomer in an aggregate expressed as a function of e.g. the number of
first neighbors of that monomer.

The Langevin thermostat has been constructed as is to model the
statistical NVT ensemble, while still being very simple.
I think that if you would modify the Langevin thermostat in that way
without exactly knowing what you're doing, you would mess up this
statistical ensemble that your simulation lives in, and I wouldn't
expect that any of the effects that you might see in such a simulation
has any resemblance to what happens in nature.

As for whether there might be useful alternatives for you, I think that
others on this list have a better founded opinion on this matter.

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Dipl.-Phys. Christoph Junghans
Max Planck Institute for Polymer Research
Theory Group
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