LAMMPS WWW Site - LAMMPS Documentation - LAMMPS Commands
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.
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 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
- density = mass/volume, where rho* = rho sigma^dim
Note that for LJ units, the default mode of thermodyamic 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/mole
- velocity = Angstroms/femtosecond
- force = Kcal/mole-Angstrom
- torque = Kcal/mole
- 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 = gram/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 fmsec and skin = 2.0 Angstroms.
- For style metal these are dt = 0.001 psec and skin = 2.0 Angstroms.
- For style si these are dt = 1.0e-8 sec and skin = 0.001 meters.
- For style cgs these are dt = 1.0e-8 sec and skin = 0.1 cm.
- For style electron these are dt = 0.001 fmsec and skin = 2.0 Bohr.
- For style micro these are dt = 2.0 microsec and skin = 0.1 micrometers.
- For style nano these are dt = 0.00045 nanosec 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