[lammps-users] Mapping LJ unit and Langevin parameters to a real system

Dear Lammps Community,

I would like to map the LJ and Langevin parameters to a real. By using a=100 nm, T=298 K, and m=18.54*10^4 Da, I get epsilon=4.11 zJ and tau=27.34 ns. However, I do not know how to appropriately mapping friction coefficient. Based on Lammps’s documentation and dimension analysis, for damp=10 and a dt=0.005, I get the following results:

dt = 0.00527.35 ns = 0.136 ns,
damping constant = 1/(damping factor
tau) = 3.6510^6 s^-1,
friction coefficient = mass
damping constant = 1.226 fkg / s,
diffusion coefficient = epsilon/friction coefficient =3.657 mm^2/s,
viscosity = 1.19 nKg/(m.s) — Based on Stokes-Einstein formula for a sphere,

all measured in SI units.

Are my conversion formula for damping constant correct and thus the subsequent results correct?

I also have one further question about the meaning of damping factor: If we set damp=10, do we expect to see diffusive behaviour approximately in time steps larger than damp/dt? In my example above, time steps >> 10/0.005 (or time steps >> 2000)?