units command — LAMMPS documentation

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# units command

## Syntax

    units style

- style = lj or real or metal or si or cgs or electron or micro or nano

## Examples

    units metal
    units lj

## Description

This command sets the style of units used for a simulation. It determines the units of all quantities specified in the input script and data file, as well as quantities output to the screen, log file, and dump files. Typically, this command is used at the very beginning of an input script.

For all units except lj, LAMMPS uses physical constants from www.physics.nist.gov. For the definition of kcal in real units, LAMMPS uses the thermochemical calorie = 4.184 J.

The choice you make for units simply sets some internal conversion factors within LAMMPS. This means that any simulation you perform for one choice of units can be duplicated with any other unit setting LAMMPS supports. In this context “duplicate” means the particles will have identical trajectories and all output generated by the simulation will be identical. This will be the case for some number of timesteps until round-off effects accumulate, since the conversion factors for two different unit systems are not identical to infinite precision.

To perform the same simulation in a different set of units you must change all the unit-based input parameters in your input script and other input files (data file, potential files, etc) correctly to the new units. And you must correctly convert all output from the new units to the old units when comparing to the original results. That is often not simple to do.

Potential or table files may have a `UNITS:` tag included in the first line indicating the unit style those files were created for. If the tag exists, its value will be compared to the chosen unit style and LAMMPS will stop with an error message if there is a mismatch. In some select cases and for specific combinations of unit styles, LAMMPS is capable of automatically converting potential parameters from a file. In those cases, a warning message signaling that an automatic conversion has happened is printed to the screen.

For style lj, all quantities are unitless. Without loss of generality, LAMMPS sets the fundamental quantities mass, \(\sigma\), \(\epsilon\), and the Boltzmann constant \(k_B = 1\). 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, \(\sigma\), and \(\epsilon\) values for a specific material and convert the results from a unitless LJ simulation into physical quantities. Please note that using these three properties as base, your unit of time has to conform to the relation \(\epsilon = \frac{m \sigma^2}{\tau^2}\) since energy is a derived unit (in SI units you equivalently have the relation \(1\mathsf{J} = 1\frac{\mathsf{kg}\cdot\mathsf{m}^2}{\mathsf{s}^2}\)).

- mass = mass or \(m\), where \(M^* = \frac{M}{m}\)

- distance = \(\sigma\), where \(x^* = \frac{x}{\sigma}\)

- time = \(\tau\), where \(\tau^* = \tau \sqrt{\frac{\epsilon}{m \sigma^2}}\)

- energy = \(\epsilon\), where \(E^* = \frac{E}{\epsilon}\)

- velocity = \(\frac{\sigma}{\tau}\), where \(v^* = v \frac{\tau}{\sigma}\)

- force = \(\frac{\epsilon}{\sigma}\), where \(f^* = f \frac{\sigma}{\epsilon}\)

- torque = \(\epsilon\), where \(t^* = \frac{t}{\epsilon}\)

- temperature = reduced LJ temperature, where \(T^* = \frac{T k_B}{\epsilon}\)

- pressure = reduced LJ pressure, where \(p^* = p \frac{\sigma^3}{\epsilon}\)

- dynamic viscosity = reduced LJ viscosity, where \(\eta^* = \eta \frac{\sigma^3}{\epsilon\tau}\)

- charge = reduced LJ charge, where \(q^* = q \frac{1}{\sqrt{4 \pi \varepsilon_0 \sigma \epsilon}}\)

- dipole = reduced LJ dipole, moment where \(\mu^* = \mu \frac{1}{\sqrt{4 \pi \varepsilon_0 \sigma^3 \epsilon}}\)

- electric field = force/charge, where \(E^* = E \frac{\sqrt{4 \pi \varepsilon_0 \sigma \epsilon} \sigma}{\epsilon}\)

- density = mass/volume, where \(\rho^* = \rho \frac{\sigma^{dim}}{m}\)

Note that for LJ units, the default mode of thermodynamic output via the thermo_style command is to normalize all extensive quantities by the number of atoms. E.g. potential energy is extensive because it is summed over atoms, so it is output as energy/atom. Temperature is intensive since it is already normalized by the number of atoms, so it is output as-is. This behavior can be changed via the thermo_modify norm command.

For style real, these are the units:

- mass = grams/mole

- distance = Angstroms

- time = femtoseconds

- energy = kcal/mol

- velocity = Angstroms/femtosecond

- force = (kcal/mol)/Angstrom

- torque = kcal/mol

- temperature = Kelvin

- pressure = atmospheres

- dynamic viscosity = Poise

- charge = multiple of electron charge (1.0 is a proton)

- dipole = charge*Angstroms

- electric field = volts/Angstrom

- density = g/cm^dim

For style metal, these are the units:

- mass = grams/mole

- distance = Angstroms

- time = picoseconds

- energy = eV

- velocity = Angstroms/picosecond

- force = eV/Angstrom

- torque = eV

- temperature = Kelvin

- pressure = bars

- dynamic viscosity = Poise

- charge = multiple of electron charge (1.0 is a proton)

- dipole = charge*Angstroms

- electric field = volts/Angstrom

- density = gram/cm^dim

For style si, these are the units:

- mass = kilograms

- distance = meters

- time = seconds

- energy = Joules

- velocity = meters/second

- force = Newtons

- torque = Newton-meters

- temperature = Kelvin

- pressure = Pascals

- dynamic viscosity = Pascal*second

- charge = Coulombs (1.6021765e-19 is a proton)

- dipole = Coulombs*meters

- electric field = volts/meter

- density = kilograms/meter^dim

For style cgs, these are the units:

- mass = grams

- distance = centimeters

- time = seconds

- energy = ergs

- velocity = centimeters/second

- force = dynes

- torque = dyne-centimeters

- temperature = Kelvin

- pressure = dyne/cm^2 or barye = 1.0e-6 bars

- dynamic viscosity = Poise

- charge = statcoulombs or esu (4.8032044e-10 is a proton)

- dipole = statcoul-cm = 10^18 debye

- electric field = statvolt/cm or dyne/esu

- density = grams/cm^dim

For style electron, these are the units:

- mass = atomic mass units

- distance = Bohr

- time = femtoseconds

- energy = Hartrees

- velocity = Bohr/atomic time units [1.03275e-15 seconds]

- force = Hartrees/Bohr

- temperature = Kelvin

- pressure = Pascals

- charge = multiple of electron charge (1.0 is a proton)

- dipole moment = Debye

- electric field = volts/cm

For style micro, these are the units:

- mass = picograms

- distance = micrometers

- time = microseconds

- energy = picogram-micrometer^2/microsecond^2

- velocity = micrometers/microsecond

- force = picogram-micrometer/microsecond^2

- torque = picogram-micrometer^2/microsecond^2

- temperature = Kelvin

- pressure = picogram/(micrometer-microsecond^2)

- dynamic viscosity = picogram/(micrometer-microsecond)

- charge = picocoulombs (1.6021765e-7 is a proton)

- dipole = picocoulomb-micrometer

- electric field = volt/micrometer

- density = picograms/micrometer^dim

For style nano, these are the units:

- mass = attograms

- distance = nanometers

- time = nanoseconds

- energy = attogram-nanometer^2/nanosecond^2

- velocity = nanometers/nanosecond

- force = attogram-nanometer/nanosecond^2

- torque = attogram-nanometer^2/nanosecond^2

- temperature = Kelvin

- pressure = attogram/(nanometer-nanosecond^2)

- dynamic viscosity = attogram/(nanometer-nanosecond)

- charge = multiple of electron charge (1.0 is a proton)

- dipole = charge-nanometer

- electric field = volt/nanometer

- density = attograms/nanometer^dim

The units command also sets the timestep size and neighbor skin distance to default values for each style:

- For style lj these are dt = 0.005 \(\tau\) and skin = 0.3 \(\sigma\).

- For style real these are dt = 1.0 femtoseconds and skin = 2.0 Angstroms.

- For style metal these are dt = 0.001 picoseconds and skin = 2.0 Angstroms.

- For style si these are dt = 1.0e-8 seconds and skin = 0.001 meters.

- For style cgs these are dt = 1.0e-8 seconds and skin = 0.1 centimeters.

- For style electron these are dt = 0.001 femtoseconds and skin = 2.0 Bohr.

- For style micro these are dt = 2.0 microseconds and skin = 0.1 micrometers.

- For style nano these are dt = 0.00045 nanoseconds and skin = 0.1 nanometers.

## Restrictions

This command cannot be used after the simulation box is defined by a read_data or create_box command.

## Related commands

none

## Default

    units lj