Hello everybody,

I am simulating a system of polymers and nanoparticles (NPs) using brownian dynamics implemented via fix nve + fix langevin.

At the beginning, I set a single ‘damp’ parameter for all the species in the systems (monomers and NPs), but then I realized that by doing so the viscosity of the system will not be well-defined, because the small monomers and the large NPs will experience the same friction, and for the Stokes law the fritction coefficient (m/damp) should be proportional to the radius of the particle.

I then decided to scale the ‘damp’ parameter with a factor m/R (m=mass, R=radius) using the ‘scale’ keyword of fix langevin. This should give a well-defined, unique viscosity to the implicit solvent I am simulating. To be more precise, a free monomer and a free NP freely diffusing now satisfy the Stokes-Einstein relation with the same value of the viscosity (which I verified).

But then, reading many articles about simulations of polydisperse systems, many of which realized with LAMMPS, I found no mention of the need to use different ‘damp’ parameters for particles of different size when using a langevin thermostat.

So I began wondering: is my reasoning correct?

Thank you very much,

Valerio