forked from lijiext/lammps
139 lines
4.1 KiB
Plaintext
139 lines
4.1 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Section_commands.html#comm)
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:line
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units command :h3
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[Syntax:]
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units style :pre
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style = {lj} or {real} or {metal} or {si} or {cgs} :ul
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[Examples:]
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units metal
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units lj :pre
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[Description:]
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This command sets the style of units used for a simulation. It
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determines the units of all quantities specified in the input script
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and data file, as well as quantities output to the screen, log file,
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and dump files. Typically, this command is used at the very beginning
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of an input script.
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For all units except {lj}, LAMMPS uses physical constants from
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www.physics.nist.gov. For the definition of Kcal in real units,
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LAMMPS uses the thermochemical calorie = 4.184 J.
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For style {lj}, all quantities are unitless. Without loss of
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generality, LAMMPS sets the fundamental quantities mass, sigma,
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epsilon, and the Boltzmann constant = 1. The masses, distances,
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energies you specify are multiples of these fundamental values. The
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formulas relating the reduced or unitless quantity (with an asterisk)
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to the same quantity with units is also given. Thus you can use the
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mass & sigma & epsilon values for a specific material and convert the
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results from a unitless LJ simulation into physical quantities.
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mass = mass or m
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distance = sigma, where x* = x / sigma
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time = tau, where tau = t* = t (epsilon / m / sigma^2)^1/2
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energy = epsilon, where E* = E / epsilon
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velocity = sigma/tau, where v* = v tau / sigma
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force = epsilon/sigma, where f* = f sigma / epsilon
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torque = epsilon, where t* = t / epsilon
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temperature = reduced LJ temperature, where T* = T Kb / epsilon
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pressure = reduced LJ pressure, where P* = P sigma^3 / epsilon
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viscosity = reduced LJ viscosity, where eta* = eta sigma^3 / epsilon / tau
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charge = reduced LJ charge, where q* = q / (4 pi perm0 sigma epsilon)^1/2
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dipole = reduced LJ dipole, moment where *mu = mu / (4 pi perm0 sigma^3 epsilon)^1/2
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electric field = force/charge, where E* = E (4 pi perm0 sigma epsilon)^1/2 sigma / epsilon :ul
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For style {real}, these are the units:
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mass = grams/mole
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distance = Angstroms
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time = femtoseconds
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energy = Kcal/mole
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velocity = Angstroms/femtosecond
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force = Kcal/mole-Angstrom
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torque = Kcal/mole
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temperature = degrees K
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pressure = atmospheres
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viscosity = Poise
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charge = multiple of electron charge (+1.0 is a proton)
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dipole = charge*Angstroms
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electric field = volts/Angstrom :ul
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For style {metal}, these are the units:
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mass = grams/mole
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distance = Angstroms
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time = picoseconds
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energy = eV
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velocity = Angstroms/picosecond
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force = eV/Angstrom
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torque = eV
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temperature = degrees K
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pressure = bars
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viscosity = Poise
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charge = multiple of electron charge (+1.0 is a proton)
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dipole = charge*Angstroms
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electric field = volts/Angstrom :ul
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For style {si}, these are the units:
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mass = kilograms
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distance = meters
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time = seconds
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energy = Joules
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velocity = meters/second
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force = Newtons
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torque = Newton-meters
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temperature = degrees K
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pressure = Pascals
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viscosity = Pascal*second
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charge = Coulombs
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dipole = Coulombs*meters
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electric field = volts/meter :ul
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For style {cgs}, these are the units:
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mass = grams
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distance = centimeters
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time = seconds
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energy = ergs
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velocity = centimeters/second
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force = dynes
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torque = dyne-centimeters
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temperature = degrees K
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pressure = dyne/cm^2 or barye = 1.0e-6 bars
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viscosity = Poise
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charge = statcoulombs or esu
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dipole = statcoul-cm = 10^18 debye
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electric field = statvolt/cm or dyne/esu :ul
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The units command also sets the timestep size and neighbor skin
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distance to default values for each style. For style {lj} these are
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dt = 0.005 tau and skin = 0.3 sigma. For style {real} these are dt =
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1.0 fmsec and skin = 2.0 Angstroms. For style {metal} these are dt =
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0.001 psec and skin = 2.0 Angstroms. For style {si} these are dt =
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1.0e-8 sec and skin = 0.001 meters. For style {cgs} these are dt =
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1.0e-8 sec and skin = 0.1 cm.
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[Restrictions:]
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This command cannot be used after the simulation box is defined by a
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"read_data"_read_data.html or "create_box"_create_box.html command.
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[Related commands:] none
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[Default:]
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units lj :pre
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