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.. index:: units
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units command
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=============
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Syntax
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""""""
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.. parsed-literal::
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units style
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* style = *lj* or *real* or *metal* or *si* or *cgs* or *electron* or *micro* or *nano*
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Examples
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""""""""
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.. parsed-literal::
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units metal
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units lj
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Description
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"""""""""""
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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
|
||||
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.
|
||||
|
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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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|
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The choice you make for units simply sets some internal conversion
|
||||
factors within LAMMPS. This means that any simulation you perform for
|
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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.
|
||||
|
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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
|
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other input files (data file, potential files, etc) correctly to the
|
||||
new units. And you must correctly convert all output from the new
|
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units to the old units when comparing to the original results. That
|
||||
is often not simple to do.
|
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|
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----------
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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 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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* dynamic 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
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* density = mass/volume, where rho* = rho sigma^dim
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Note that for LJ units, the default mode of thermodyamic output via
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the :doc:`thermo_style <thermo_style>` command is to normalize all
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extensive quantities by the number of atoms. E.g. potential energy is
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extensive because it is summed over atoms, so it is output as
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energy/atom. Temperature is intensive since it is already normalized
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by the number of atoms, so it is output as-is. This behavior can be
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changed via the :doc:`thermo_modify norm <thermo_modify>` command.
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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 = Kelvin
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* pressure = atmospheres
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* dynamic 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
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* density = gram/cm^dim
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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 = Kelvin
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* pressure = bars
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* dynamic 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
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* density = gram/cm^dim
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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 = Kelvin
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* pressure = Pascals
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* dynamic viscosity = Pascal*second
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* charge = Coulombs (1.6021765e-19 is a proton)
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* dipole = Coulombs*meters
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* electric field = volts/meter
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* density = kilograms/meter^dim
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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 = Kelvin
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* pressure = dyne/cm^2 or barye = 1.0e-6 bars
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* dynamic viscosity = Poise
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* charge = statcoulombs or esu (4.8032044e-10 is a proton)
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* dipole = statcoul-cm = 10^18 debye
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* electric field = statvolt/cm or dyne/esu
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* density = grams/cm^dim
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For style *electron*\ , these are the units:
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* mass = atomic mass units
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* distance = Bohr
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* time = femtoseconds
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* energy = Hartrees
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* velocity = Bohr/atomic time units [1.03275e-15 seconds]
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* force = Hartrees/Bohr
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* temperature = Kelvin
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* pressure = Pascals
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* charge = multiple of electron charge (1.0 is a proton)
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* dipole moment = Debye
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* electric field = volts/cm
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For style *micro*\ , these are the units:
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* mass = picograms
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* distance = micrometers
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* time = microseconds
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* energy = picogram-micrometer^2/microsecond^2
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* velocity = micrometers/microsecond
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* force = picogram-micrometer/microsecond^2
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* torque = picogram-micrometer^2/microsecond^2
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* temperature = Kelvin
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* pressure = picogram/(micrometer-microsecond^2)
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* dynamic viscosity = picogram/(micrometer-microsecond)
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* charge = picocoulombs (1.6021765e-7 is a proton)
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* dipole = picocoulomb-micrometer
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* electric field = volt/micrometer
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* density = picograms/micrometer^dim
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For style *nano*\ , these are the units:
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* mass = attograms
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* distance = nanometers
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* time = nanoseconds
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* energy = attogram-nanometer^2/nanosecond^2
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* velocity = nanometers/nanosecond
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* force = attogram-nanometer/nanosecond^2
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* torque = attogram-nanometer^2/nanosecond^2
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* temperature = Kelvin
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* pressure = attogram/(nanometer-nanosecond^2)
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* dynamic viscosity = attogram/(nanometer-nanosecond)
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* charge = multiple of electron charge (1.0 is a proton)
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* dipole = charge-nanometer
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* electric field = volt/nanometer
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* density = attograms/nanometer^dim
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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:
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* For style *lj* these are dt = 0.005 tau and skin = 0.3 sigma.
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* For style *real* these are dt = 1.0 fmsec and skin = 2.0 Angstroms.
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* For style *metal* these are dt = 0.001 psec and skin = 2.0 Angstroms.
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* For style *si* these are dt = 1.0e-8 sec and skin = 0.001 meters.
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* For style *cgs* these are dt = 1.0e-8 sec and skin = 0.1 cm.
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* For style *electron* these are dt = 0.001 fmsec and skin = 2.0 Bohr.
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* For style *micro* these are dt = 2.0 microsec and skin = 0.1 micrometers.
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* For style *nano* these are dt = 0.00045 nanosec and skin = 0.1 nanometers.
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Restrictions
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""""""""""""
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This command cannot be used after the simulation box is defined by a
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:doc:`read_data <read_data>` or :doc:`create_box <create_box>` command.
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**Related commands:** none
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Default
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"""""""
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.. parsed-literal::
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units lj
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.. _lws: http://lammps.sandia.gov
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.. _ld: Manual.html
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.. _lc: Section_commands.html#comm
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|
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<li class="toctree-l1"><a class="reference internal" href="Section_intro.html">1. Introduction</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_commands.html">3. Commands</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_packages.html">4. Packages</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_accelerate.html">5. Accelerating LAMMPS performance</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_example.html">7. Example problems</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_history.html">13. Future and history</a></li>
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<li>units command</li>
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||||
<div class="section" id="units-command">
|
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<span id="index-0"></span><h1>units command</h1>
|
||||
<div class="section" id="syntax">
|
||||
<h2>Syntax</h2>
|
||||
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">units</span> <span class="n">style</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
<ul class="simple">
|
||||
<li>style = <em>lj</em> or <em>real</em> or <em>metal</em> or <em>si</em> or <em>cgs</em> or <em>electron</em> or <em>micro</em> or <em>nano</em></li>
|
||||
</ul>
|
||||
</div>
|
||||
<div class="section" id="examples">
|
||||
<h2>Examples</h2>
|
||||
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">units</span> <span class="n">metal</span>
|
||||
<span class="n">units</span> <span class="n">lj</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
</div>
|
||||
<div class="section" id="description">
|
||||
<h2>Description</h2>
|
||||
<p>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.</p>
|
||||
<p>For all units except <em>lj</em>, 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.</p>
|
||||
<p>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.</p>
|
||||
<p>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.</p>
|
||||
<hr class="docutils" />
|
||||
<p>For style <em>lj</em>, 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.</p>
|
||||
<ul class="simple">
|
||||
<li>mass = mass or m</li>
|
||||
<li>distance = sigma, where x* = x / sigma</li>
|
||||
<li>time = tau, where t* = t (epsilon / m / sigma^2)^1/2</li>
|
||||
<li>energy = epsilon, where E* = E / epsilon</li>
|
||||
<li>velocity = sigma/tau, where v* = v tau / sigma</li>
|
||||
<li>force = epsilon/sigma, where f* = f sigma / epsilon</li>
|
||||
<li>torque = epsilon, where t* = t / epsilon</li>
|
||||
<li>temperature = reduced LJ temperature, where T* = T Kb / epsilon</li>
|
||||
<li>pressure = reduced LJ pressure, where P* = P sigma^3 / epsilon</li>
|
||||
<li>dynamic viscosity = reduced LJ viscosity, where eta* = eta sigma^3 / epsilon / tau</li>
|
||||
<li>charge = reduced LJ charge, where q* = q / (4 pi perm0 sigma epsilon)^1/2</li>
|
||||
<li>dipole = reduced LJ dipole, moment where <a href="#id1"><span class="problematic" id="id2">*</span></a>mu = mu / (4 pi perm0 sigma^3 epsilon)^1/2</li>
|
||||
<li>electric field = force/charge, where E* = E (4 pi perm0 sigma epsilon)^1/2 sigma / epsilon</li>
|
||||
<li>density = mass/volume, where rho* = rho sigma^dim</li>
|
||||
</ul>
|
||||
<p>Note that for LJ units, the default mode of thermodyamic output via
|
||||
the <a class="reference internal" href="thermo_style.html"><span class="doc">thermo_style</span></a> 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 <a class="reference internal" href="thermo_modify.html"><span class="doc">thermo_modify norm</span></a> command.</p>
|
||||
<p>For style <em>real</em>, these are the units:</p>
|
||||
<ul class="simple">
|
||||
<li>mass = grams/mole</li>
|
||||
<li>distance = Angstroms</li>
|
||||
<li>time = femtoseconds</li>
|
||||
<li>energy = Kcal/mole</li>
|
||||
<li>velocity = Angstroms/femtosecond</li>
|
||||
<li>force = Kcal/mole-Angstrom</li>
|
||||
<li>torque = Kcal/mole</li>
|
||||
<li>temperature = Kelvin</li>
|
||||
<li>pressure = atmospheres</li>
|
||||
<li>dynamic viscosity = Poise</li>
|
||||
<li>charge = multiple of electron charge (1.0 is a proton)</li>
|
||||
<li>dipole = charge*Angstroms</li>
|
||||
<li>electric field = volts/Angstrom</li>
|
||||
<li>density = gram/cm^dim</li>
|
||||
</ul>
|
||||
<p>For style <em>metal</em>, these are the units:</p>
|
||||
<ul class="simple">
|
||||
<li>mass = grams/mole</li>
|
||||
<li>distance = Angstroms</li>
|
||||
<li>time = picoseconds</li>
|
||||
<li>energy = eV</li>
|
||||
<li>velocity = Angstroms/picosecond</li>
|
||||
<li>force = eV/Angstrom</li>
|
||||
<li>torque = eV</li>
|
||||
<li>temperature = Kelvin</li>
|
||||
<li>pressure = bars</li>
|
||||
<li>dynamic viscosity = Poise</li>
|
||||
<li>charge = multiple of electron charge (1.0 is a proton)</li>
|
||||
<li>dipole = charge*Angstroms</li>
|
||||
<li>electric field = volts/Angstrom</li>
|
||||
<li>density = gram/cm^dim</li>
|
||||
</ul>
|
||||
<p>For style <em>si</em>, these are the units:</p>
|
||||
<ul class="simple">
|
||||
<li>mass = kilograms</li>
|
||||
<li>distance = meters</li>
|
||||
<li>time = seconds</li>
|
||||
<li>energy = Joules</li>
|
||||
<li>velocity = meters/second</li>
|
||||
<li>force = Newtons</li>
|
||||
<li>torque = Newton-meters</li>
|
||||
<li>temperature = Kelvin</li>
|
||||
<li>pressure = Pascals</li>
|
||||
<li>dynamic viscosity = Pascal*second</li>
|
||||
<li>charge = Coulombs (1.6021765e-19 is a proton)</li>
|
||||
<li>dipole = Coulombs*meters</li>
|
||||
<li>electric field = volts/meter</li>
|
||||
<li>density = kilograms/meter^dim</li>
|
||||
</ul>
|
||||
<p>For style <em>cgs</em>, these are the units:</p>
|
||||
<ul class="simple">
|
||||
<li>mass = grams</li>
|
||||
<li>distance = centimeters</li>
|
||||
<li>time = seconds</li>
|
||||
<li>energy = ergs</li>
|
||||
<li>velocity = centimeters/second</li>
|
||||
<li>force = dynes</li>
|
||||
<li>torque = dyne-centimeters</li>
|
||||
<li>temperature = Kelvin</li>
|
||||
<li>pressure = dyne/cm^2 or barye = 1.0e-6 bars</li>
|
||||
<li>dynamic viscosity = Poise</li>
|
||||
<li>charge = statcoulombs or esu (4.8032044e-10 is a proton)</li>
|
||||
<li>dipole = statcoul-cm = 10^18 debye</li>
|
||||
<li>electric field = statvolt/cm or dyne/esu</li>
|
||||
<li>density = grams/cm^dim</li>
|
||||
</ul>
|
||||
<p>For style <em>electron</em>, these are the units:</p>
|
||||
<ul class="simple">
|
||||
<li>mass = atomic mass units</li>
|
||||
<li>distance = Bohr</li>
|
||||
<li>time = femtoseconds</li>
|
||||
<li>energy = Hartrees</li>
|
||||
<li>velocity = Bohr/atomic time units [1.03275e-15 seconds]</li>
|
||||
<li>force = Hartrees/Bohr</li>
|
||||
<li>temperature = Kelvin</li>
|
||||
<li>pressure = Pascals</li>
|
||||
<li>charge = multiple of electron charge (1.0 is a proton)</li>
|
||||
<li>dipole moment = Debye</li>
|
||||
<li>electric field = volts/cm</li>
|
||||
</ul>
|
||||
<p>For style <em>micro</em>, these are the units:</p>
|
||||
<ul class="simple">
|
||||
<li>mass = picograms</li>
|
||||
<li>distance = micrometers</li>
|
||||
<li>time = microseconds</li>
|
||||
<li>energy = picogram-micrometer^2/microsecond^2</li>
|
||||
<li>velocity = micrometers/microsecond</li>
|
||||
<li>force = picogram-micrometer/microsecond^2</li>
|
||||
<li>torque = picogram-micrometer^2/microsecond^2</li>
|
||||
<li>temperature = Kelvin</li>
|
||||
<li>pressure = picogram/(micrometer-microsecond^2)</li>
|
||||
<li>dynamic viscosity = picogram/(micrometer-microsecond)</li>
|
||||
<li>charge = picocoulombs (1.6021765e-7 is a proton)</li>
|
||||
<li>dipole = picocoulomb-micrometer</li>
|
||||
<li>electric field = volt/micrometer</li>
|
||||
<li>density = picograms/micrometer^dim</li>
|
||||
</ul>
|
||||
<p>For style <em>nano</em>, these are the units:</p>
|
||||
<ul class="simple">
|
||||
<li>mass = attograms</li>
|
||||
<li>distance = nanometers</li>
|
||||
<li>time = nanoseconds</li>
|
||||
<li>energy = attogram-nanometer^2/nanosecond^2</li>
|
||||
<li>velocity = nanometers/nanosecond</li>
|
||||
<li>force = attogram-nanometer/nanosecond^2</li>
|
||||
<li>torque = attogram-nanometer^2/nanosecond^2</li>
|
||||
<li>temperature = Kelvin</li>
|
||||
<li>pressure = attogram/(nanometer-nanosecond^2)</li>
|
||||
<li>dynamic viscosity = attogram/(nanometer-nanosecond)</li>
|
||||
<li>charge = multiple of electron charge (1.0 is a proton)</li>
|
||||
<li>dipole = charge-nanometer</li>
|
||||
<li>electric field = volt/nanometer</li>
|
||||
<li>density = attograms/nanometer^dim</li>
|
||||
</ul>
|
||||
<p>The units command also sets the timestep size and neighbor skin
|
||||
distance to default values for each style:</p>
|
||||
<ul class="simple">
|
||||
<li>For style <em>lj</em> these are dt = 0.005 tau and skin = 0.3 sigma.</li>
|
||||
<li>For style <em>real</em> these are dt = 1.0 fmsec and skin = 2.0 Angstroms.</li>
|
||||
<li>For style <em>metal</em> these are dt = 0.001 psec and skin = 2.0 Angstroms.</li>
|
||||
<li>For style <em>si</em> these are dt = 1.0e-8 sec and skin = 0.001 meters.</li>
|
||||
<li>For style <em>cgs</em> these are dt = 1.0e-8 sec and skin = 0.1 cm.</li>
|
||||
<li>For style <em>electron</em> these are dt = 0.001 fmsec and skin = 2.0 Bohr.</li>
|
||||
<li>For style <em>micro</em> these are dt = 2.0 microsec and skin = 0.1 micrometers.</li>
|
||||
<li>For style <em>nano</em> these are dt = 0.00045 nanosec and skin = 0.1 nanometers.</li>
|
||||
</ul>
|
||||
</div>
|
||||
<div class="section" id="restrictions">
|
||||
<h2>Restrictions</h2>
|
||||
<p>This command cannot be used after the simulation box is defined by a
|
||||
<a class="reference internal" href="read_data.html"><span class="doc">read_data</span></a> or <a class="reference internal" href="create_box.html"><span class="doc">create_box</span></a> command.</p>
|
||||
<p><strong>Related commands:</strong> none</p>
|
||||
</div>
|
||||
<div class="section" id="default">
|
||||
<h2>Default</h2>
|
||||
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">units</span> <span class="n">lj</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
</div>
|
||||
</div>
|
||||
|
||||
|
||||
</div>
|
||||
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|
||||
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|
||||
|
||||
|
||||
<hr/>
|
||||
|
||||
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|
||||
<p>
|
||||
© Copyright 2013 Sandia Corporation.
|
||||
</p>
|
||||
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|
||||
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|
||||
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||||
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|
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|
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|
||||
|
||||
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|
||||
|
||||
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|
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|
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|
||||
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Reference in New Issue