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
+