\(\renewcommand{\AA}{\text{Å}}\)
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.
Default
units lj