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, LIGGGHTS(R)-PUBLIC uses physical constants from www.physics.nist.gov. For the definition of Kcal in real units, LIGGGHTS(R)-PUBLIC uses the thermochemical calorie = 4.184 J.

For style lj, all quantities are unitless. Without loss of generality, LIGGGHTS(R)-PUBLIC 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 tau = 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 energies by the number of atoms, i.e. energy/atom. This 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
• 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
• 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
• 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