Unit Systems
The unit systems in LAMMPS can be found here. In addition to SI units, we also use the two unit systems described below.
The cgs units assume ESU, but it can also be used for magnetostatics.
The CGS EMU units are provided below. Note that there are many different conventions for
Gaussian units. We use this.
CGS EMU
| Quantity | Unit | SI |
|---|---|---|
| Length | cm | \(10^{-2}\) m |
| Time | s | 1 s |
| Mass | g | \(10^{-3}\) kg |
| Force | dyne | \(10^{-5}\) N |
| Energy | erg | \(10^{-7}\) J |
| Density | g/cm\(^3\) | \(10^3\) kg/m\(^3\) |
| Stress | Ba | \(10^{-1}\) Pa |
| Magnetic dipole | emu = erg/G | \(10^{-3}\) A m\(^2\) |
| Magnetic field | G | \(10^{-4}\) T |
| Kinematic viscosity | cm\(^2\)/s | \(10^{-4}\) m\(^2\)/s |
| Temperature | K | 1 K |
A custom milli unit system (mm, mg, ms, \(\mu_0/4\pi = 1\)) is also used in some examples. All other units except temperature can be derived from the four fundamental units.
Note
The temperature in a DEM simulation is only used as a proxy for the kinetic energy
of the particles, relevant only when a Langevin thermostat is applied for quasi-static equilibration.
Its scale is set by \(\{k_B\}\), the numerical value of the Boltzmann constant in the chosen LAMMPS unit system
(\(\{k_B\} = 1\) in units lj; \(\{k_B\} = 1.38 \times 10^{-16}\) in units cgs).
milli
| Quantity | Unit | SI |
|---|---|---|
| Length | mm | \(10^{-3}\) m |
| Time | ms | \(10^{-3}\) s |
| Mass | mg | \(10^{-6}\) kg |
| Force | mN | \(10^{-3}\) N |
| Energy | \(\mu\)J | \(10^{-6}\) J |
| Density | mg/mm\(^3\) | \(10^3\) kg/m\(^3\) |
| Stress | kPa | \(10^3\) Pa |
| Magnetic dipole | -- | \(10^{-4}\) A m\(^2\) |
| Magnetic field | -- | \(10^{-2}\) T |
| Kinematic viscosity | mm\(^2\)/ms | \(10^{-3}\) m\(^2\)/s |
| Temperature | -- | \(10^{-6}/\{k_B\}\) K |