+
+ +
+

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
+
+
+
+
+ + +
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