Greetings,

I had a question related to finding the correct damping parameter for a

given system when thermostating (specifically I am using fix langevin).

The damping should be related to the effective viscosity of whatever

material you are investigating. And it seems that a correct damping

parameter should exist for a given material at a given temperature. In the

discussion on fix viscous, the damping force F is given by F = - gamma *

velocity, where gamma is a kind of friction coefficient. It is also stated

that "the gamma of fix viscous is related to the damping parameter of fix

langevin, except that the units of gamma are force/velocity and the units

of damp are time, so that it can more easily be used as a thermostat."

My question is how specifically are the two related? Since force/velocity

units end up as mass/time, and the langevin damping is in time. Assuming I

can get an accurate gamma, I would like to get an accurate langevin damping

parameter.

I could use a standard damping of 10, or 100, or something that just works.

However, I should be able to better model my intended system knowing the

actual damping parameter.

Any help is most appreciated, and if this doesnt make sense please let me

know.

Thanks all.

Ben

Benjamin Warren Beeler

Mechanical and Aerospace Engineering

University of California, Davis