# brownian dynamics in lammps

Please reply to the mailing list, not to me. Also, I am not Anna.

If your spheres have a density of 3.36E-20 and a diameter of 1, they have a mass of 0.25Pi3.36E-20, which is roughly 2.6E-20.

Then you ask LAMMPS for a temperature of of 0.069.

Since 0.5mv^2 should give 3/2 * kT, this means <v^2> should end up at about 3 * 0.069 / 2.6E-20 which is roughly 8 E+18. This is extremely large. With your current time step size, it means that a particle can jump through the box boundaries a couple of hundred times each time step, which is clearly not what should be happening.

I donâ€™t know how you came up with this low density, but I am fairly confident that it is wrong.

Thanks for your help, I am sorry to make some mistakes for replying and name!
The density is not the real density, it is the reduced density. The real density is 4200 Kg/m3, the diameter of my spheres is 2e-8 m, so
density= mass/volume, where rho*=rho sigma^dim= 4200*(2e-8)^3=3.36e-20. if it is right? please tell me. Thanks very much!

best wishes

If you make the actual diameter of your particles of 2e-8 m equal to 1 sigma in Lennard-Jones units, then you cannot just supply the density in SI units. You have to properly map your real units to sensible Lennard-Jones units and stick to those everywhere in your input scripts. Maybe you should speak to your local simulation expert/theorist on how to map your system to proper dimensionless numbers.

Hi, stefan, thank you very much, i will look over the simulation files again. best wishes for you!