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### units command

Syntax:

    units style

- style = lj or real or metal

Examples:

    units metal
    units lj

Description:

This command sets the style of units used for a simulation. It detemines 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 real and metallic units, 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.

For style lj, all quantities are unitless:

- distance = sigma
- time = tau
- mass = one
- energy = epsilon
- velocity = sigma/tau
- force = epsilon/sigma
- temperature = reduced LJ temperature
- pressure = reduced LJ pressure
- charge = reduced LJ charge
- dipole = reduced LJ dipole moment
- electric field = force/charge

For style real, these are the units:

- distance = Angstroms
- time = femtoseconds
- mass = grams/mole
- energy = Kcal/mole
- velocity = Angstroms/femtosecond
- force = Kcal/mole-Angstrom
- temperature = degrees K
- pressure = atmospheres
- charge = multiple of electron charge (+1.0 is a proton)
- dipole = charge*Angstroms
- electric field = volts/Angstrom

For style metal, these are the units:

- distance = Angstroms
- time = picoseconds
- mass = grams/mole
- energy = eV
- velocity = Angstroms/picosecond
- force = eV/Angstrom
- temperature = degrees K
- pressure = bars
- charge = multiple of electron charge (+1.0 is a proton)
- dipole = charge*Angstroms
- electric field = volts/Angstrom

This 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.

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