Re: [ESPResSo-users] No conservation of momentum

[Ulf Schiller]

Marco Blanco wrote:
> Dear all,
> 
> I'm doing a simulation on an NVT ensemble with a single polymer chain
> centered in the simulation box whose half-length is considerable larger
> than the maximum length of the polymer, so the chain doesn't interact
> with periodic images. I'm also setting the initial velocity of each
> monomer at 0, thus the initial momentum is zero too, and I'm not
> including any external force. From such initial setting, one expects
> that the net velocity of the system remains zero through the entire
> simulation, and the center of mass of the molecule doesn't move at all,
> only that the molecule folds/unfolds and rotates. However, when I run my
> simulation with Espresso, that doesn't happen. At the beginning I
> thought that it was because of the cut-off radius, so I set r_cut to a
> distance larger than the maximum length of the polymer, but the result
> was the same. I'm not very familiar with Langevin thermostats, so I
> don't know if the problem is my initial setting or if it is a
> pathological issue of the algorithm. I'd appreciate if someone can help
> me with this problem.
> 
> Thanks,
> Marco

Marco,

the Langevin thermostat is not momentum conserving - fluctuations of the
net momentum and diffusion of the center of mass are just what you would
expect for Brownian motion.

Regards,
Ulf

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