forked from lijiext/lammps
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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_start.html">2. Getting Started</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_accelerate.html">5. Accelerating LAMMPS performance</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_perf.html">8. Performance & scalability</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_modify.html">10. Modifying & extending LAMMPS</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_python.html">11. Python interface to LAMMPS</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_errors.html">12. Errors</a></li>
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<div role="main" class="document" itemscope="itemscope" itemtype="http://schema.org/Article">
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<div class="section" id="units-command">
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<span id="index-0"></span><h1>units command</h1>
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<div class="section" id="syntax">
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<h2>Syntax</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">units</span> <span class="n">style</span>
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</pre></div>
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</div>
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<ul class="simple">
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<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>
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</ul>
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</div>
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<div class="section" id="examples">
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<h2>Examples</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">units</span> <span class="n">metal</span>
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<span class="n">units</span> <span class="n">lj</span>
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</pre></div>
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</div>
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</div>
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<div class="section" id="description">
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<h2>Description</h2>
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<p>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.</p>
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<p>For all units except <em>lj</em>, 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.</p>
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<p>The choice you make for units simply sets some internal conversion
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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
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LAMMPS supports. In this context “duplicate” means the particles will
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have identical trajectories and all output generated by the simulation
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will be identical. This will be the case for some number of timesteps
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until round-off effects accumulate, since the conversion factors for
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two different unit systems are not identical to infinite precision.</p>
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<p>To perform the same simulation in a different set of units you must
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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
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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
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is often not simple to do.</p>
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<hr class="docutils" />
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<p>For style <em>lj</em>, 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.</p>
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<ul class="simple">
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<li>mass = mass or m</li>
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<li>distance = sigma, where x* = x / sigma</li>
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<li>time = tau, where t* = t (epsilon / m / sigma^2)^1/2</li>
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<li>energy = epsilon, where E* = E / epsilon</li>
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<li>velocity = sigma/tau, where v* = v tau / sigma</li>
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<li>force = epsilon/sigma, where f* = f sigma / epsilon</li>
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<li>torque = epsilon, where t* = t / epsilon</li>
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<li>temperature = reduced LJ temperature, where T* = T Kb / epsilon</li>
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<li>pressure = reduced LJ pressure, where P* = P sigma^3 / epsilon</li>
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<li>dynamic viscosity = reduced LJ viscosity, where eta* = eta sigma^3 / epsilon / tau</li>
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<li>charge = reduced LJ charge, where q* = q / (4 pi perm0 sigma epsilon)^1/2</li>
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<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>
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<li>electric field = force/charge, where E* = E (4 pi perm0 sigma epsilon)^1/2 sigma / epsilon</li>
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<li>density = mass/volume, where rho* = rho sigma^dim</li>
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</ul>
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<p>Note that for LJ units, the default mode of thermodyamic output via
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the <a class="reference internal" href="thermo_style.html"><span class="doc">thermo_style</span></a> 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 <a class="reference internal" href="thermo_modify.html"><span class="doc">thermo_modify norm</span></a> command.</p>
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<p>For style <em>real</em>, these are the units:</p>
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<ul class="simple">
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<li>mass = grams/mole</li>
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<li>distance = Angstroms</li>
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<li>time = femtoseconds</li>
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<li>energy = Kcal/mole</li>
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<li>velocity = Angstroms/femtosecond</li>
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<li>force = Kcal/mole-Angstrom</li>
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<li>torque = Kcal/mole</li>
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<li>temperature = Kelvin</li>
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<li>pressure = atmospheres</li>
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<li>dynamic viscosity = Poise</li>
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<li>charge = multiple of electron charge (1.0 is a proton)</li>
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<li>dipole = charge*Angstroms</li>
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<li>electric field = volts/Angstrom</li>
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<li>density = gram/cm^dim</li>
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</ul>
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<p>For style <em>metal</em>, these are the units:</p>
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<ul class="simple">
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<li>mass = grams/mole</li>
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<li>distance = Angstroms</li>
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<li>time = picoseconds</li>
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<li>energy = eV</li>
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<li>velocity = Angstroms/picosecond</li>
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<li>force = eV/Angstrom</li>
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<li>torque = eV</li>
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<li>temperature = Kelvin</li>
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<li>pressure = bars</li>
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<li>dynamic viscosity = Poise</li>
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<li>charge = multiple of electron charge (1.0 is a proton)</li>
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<li>dipole = charge*Angstroms</li>
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<li>electric field = volts/Angstrom</li>
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<li>density = gram/cm^dim</li>
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</ul>
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<p>For style <em>si</em>, these are the units:</p>
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<ul class="simple">
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<li>mass = kilograms</li>
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<li>distance = meters</li>
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<li>time = seconds</li>
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<li>energy = Joules</li>
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<li>velocity = meters/second</li>
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<li>force = Newtons</li>
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<li>torque = Newton-meters</li>
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<li>temperature = Kelvin</li>
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<li>pressure = Pascals</li>
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<li>dynamic viscosity = Pascal*second</li>
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<li>charge = Coulombs (1.6021765e-19 is a proton)</li>
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<li>dipole = Coulombs*meters</li>
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<li>electric field = volts/meter</li>
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<li>density = kilograms/meter^dim</li>
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</ul>
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<p>For style <em>cgs</em>, these are the units:</p>
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<ul class="simple">
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<li>mass = grams</li>
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<li>distance = centimeters</li>
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<li>time = seconds</li>
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<li>energy = ergs</li>
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<li>velocity = centimeters/second</li>
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<li>force = dynes</li>
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<li>torque = dyne-centimeters</li>
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<li>temperature = Kelvin</li>
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<li>pressure = dyne/cm^2 or barye = 1.0e-6 bars</li>
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<li>dynamic viscosity = Poise</li>
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<li>charge = statcoulombs or esu (4.8032044e-10 is a proton)</li>
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<li>dipole = statcoul-cm = 10^18 debye</li>
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<li>electric field = statvolt/cm or dyne/esu</li>
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<li>density = grams/cm^dim</li>
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</ul>
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<p>For style <em>electron</em>, these are the units:</p>
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<ul class="simple">
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<li>mass = atomic mass units</li>
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<li>distance = Bohr</li>
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<li>time = femtoseconds</li>
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<li>energy = Hartrees</li>
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<li>velocity = Bohr/atomic time units [1.03275e-15 seconds]</li>
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<li>force = Hartrees/Bohr</li>
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<li>temperature = Kelvin</li>
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<li>pressure = Pascals</li>
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<li>charge = multiple of electron charge (1.0 is a proton)</li>
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<li>dipole moment = Debye</li>
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<li>electric field = volts/cm</li>
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</ul>
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<p>For style <em>micro</em>, these are the units:</p>
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<ul class="simple">
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<li>mass = picograms</li>
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<li>distance = micrometers</li>
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<li>time = microseconds</li>
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<li>energy = picogram-micrometer^2/microsecond^2</li>
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<li>velocity = micrometers/microsecond</li>
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<li>force = picogram-micrometer/microsecond^2</li>
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<li>torque = picogram-micrometer^2/microsecond^2</li>
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<li>temperature = Kelvin</li>
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<li>pressure = picogram/(micrometer-microsecond^2)</li>
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<li>dynamic viscosity = picogram/(micrometer-microsecond)</li>
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<li>charge = picocoulombs (1.6021765e-7 is a proton)</li>
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<li>dipole = picocoulomb-micrometer</li>
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<li>electric field = volt/micrometer</li>
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<li>density = picograms/micrometer^dim</li>
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</ul>
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<p>For style <em>nano</em>, these are the units:</p>
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<ul class="simple">
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<li>mass = attograms</li>
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<li>distance = nanometers</li>
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<li>time = nanoseconds</li>
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<li>energy = attogram-nanometer^2/nanosecond^2</li>
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<li>velocity = nanometers/nanosecond</li>
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<li>force = attogram-nanometer/nanosecond^2</li>
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<li>torque = attogram-nanometer^2/nanosecond^2</li>
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<li>temperature = Kelvin</li>
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<li>pressure = attogram/(nanometer-nanosecond^2)</li>
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<li>dynamic viscosity = attogram/(nanometer-nanosecond)</li>
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<li>charge = multiple of electron charge (1.0 is a proton)</li>
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<li>dipole = charge-nanometer</li>
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<li>electric field = volt/nanometer</li>
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<li>density = attograms/nanometer^dim</li>
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</ul>
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<p>The units command also sets the timestep size and neighbor skin
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distance to default values for each style:</p>
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<ul class="simple">
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<li>For style <em>lj</em> these are dt = 0.005 tau and skin = 0.3 sigma.</li>
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<li>For style <em>real</em> these are dt = 1.0 fmsec and skin = 2.0 Angstroms.</li>
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<li>For style <em>metal</em> these are dt = 0.001 psec and skin = 2.0 Angstroms.</li>
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<li>For style <em>si</em> these are dt = 1.0e-8 sec and skin = 0.001 meters.</li>
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<li>For style <em>cgs</em> these are dt = 1.0e-8 sec and skin = 0.1 cm.</li>
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<li>For style <em>electron</em> these are dt = 0.001 fmsec and skin = 2.0 Bohr.</li>
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<li>For style <em>micro</em> these are dt = 2.0 microsec and skin = 0.1 micrometers.</li>
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<li>For style <em>nano</em> these are dt = 0.00045 nanosec and skin = 0.1 nanometers.</li>
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</ul>
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</div>
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<div class="section" id="restrictions">
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<h2>Restrictions</h2>
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<p>This command cannot be used after the simulation box is defined by a
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<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>
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<p><strong>Related commands:</strong> none</p>
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</div>
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<div class="section" id="default">
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<h2>Default</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">units</span> <span class="n">lj</span>
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</pre></div>
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</div>
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