[lammps-users] UNITS

Hello, i have a question.

Here is a part of manual:

For style lj, all quantities are unitless. Without loss of generality, LAMMPS sets the fundamental quantities mass, sigma, epsilon, and the Boltzmann constant = 1. The masses, distances, energies you specify are multiples of these fundamental values. The formulas relating the reduced or unitless quantity (with an asterisk) to the same quantity with units is also given. Thus you can use the mass & sigma & epsilon values for a specific material and convert the results from a unitless LJ simulation into physical quantities.

  • mass = mass or m
  • distance = sigma, where x* = x / sigma
  • time = tau, where tau = t* = t (epsilon / m / sigma^2)^1/2
  • energy = epsilon, where E* = E / epsilon
  • velocity = sigma/tau, where v* = v tau / sigma
  • force = epsilon/sigma, where f* = f sigma / epsilon
  • torque = epsilon, where t* = t / epsilon
  • temperature = reduced LJ temperature, where T* = T Kb / epsilon
  • pressure = reduced LJ pressure, where P* = P sigma^3 / epsilon
  • dynamic viscosity = reduced LJ viscosity, where eta* = eta sigma^3 / epsilon / tau
  • charge = reduced LJ charge, where q* = q / (4 pi perm0 sigma epsilon)^1/2
  • dipole = reduced LJ dipole, moment where *mu = mu / (4 pi perm0 sigma^3 epsilon)^1/2
  • electric field = force/charge, where E* = E (4 pi perm0 sigma epsilon)^1/2 sigma / epsilon
    I didn’t understand this article.

For example, i have a timestep = 0.003, what does this mean in seconds?
How do formulas work?

Thanks a lot, Sergey R.





I didn't understand this article.

For example, i have a timestep = 0.003, what does this mean in seconds?

nothing. unless you first find out how to express your
energy in joule, your mass in kilogram and your length
in meters. they are defined through your choice of epsilon
and sigma and mass in the potential parameters.

How do formulas work?

you insert numbers with the proper units and you get a result. :wink:


ok, i simulate Couette’s flow:

ok, i simulate Couette's flow:


How can i get information from this script? i mean epsilon, sigma, mass...

you don't understand the principle of reduced units yet.
it works the other way around.
as the unit documentation says, it works for generic particles.

all that the simulation program knows is that all particles
have the same mass, the same epsilon, and the same sigma.
it does not need to know the exact values. for example. if you
double the mass of each particle that would be the same as
dividing your time unit by sqrt(2).

find the corresponding parameters of mass, epsilon and sigma
for particles that you are interested in and _that_ will define what
your temperature and time will mean.

i need to know, how using this example script to get real units. What should
i do step by step?

there is only one step: don't get obsessed, but try to understand
for real what reduced units mean. experiment with them for a bit,
take your time, and then you'll get it.

also, there is nothing keeping you from adapting the input
for "real" units. if you want to study "real" systems you need
"real" parameters in any case. either before you run the simulation,
or afterwards.


Allen & Tildesley have an appendix on reduced units which is quite good.