# units command

## Syntax

    units style

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

## Examples

``` LAMMPS
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](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](thermo_modify) 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](read_data) or [create_box](create_box) command.

## Related commands

none

## Default

``` LAMMPS
units lj
```