[lammps-users] LJ units


I am attempting to calculate thermal conductivity of solid argon in an lj unit simulation. I was interested in determining what the default timestep was and how to convert know quantities (unknown units) to standard thermal conductivity units (W/m-K).

Again I am using lj units. My units of length are in sigmas (3.4e-10 m). My units of energy are in epsilons (1.67e-21 J). My heat added/subtracted is 82 epsilons per timestep. I do not know the timestep because I do not know if it is based on the 1e-12 or 1e-15 s time. In the documentation it says lj timestep is t* = t x tau, but I do not know what the t is since metal is 1e-12s and real is 1e-15 s. I am currently doing the calculation with conversions as follows:

k = ( dQ/dt)dx / (dTA)

where dQ is the energy added/subtracted (82) times the value of epsilon which gives 1.37e-19 J,
dt is default timestep times tau which gives t x 3e11 s (very large)
dx = 42.432 A
A = 1800.475 A^2
dT = 13 K

giving a thermal conductivity of k = 8.36e-9 W/m-K.

This result is much less than what I have obtained with other MD codes. Is there something wrong with my calculation?


Nick Roberts

If your timestep is 0.005 (default in LJ units), that
means it is 0.005 * tau. As James said the unitless
tau is t * (epsilon/m/sigma^2)^1/2.