forked from lijiext/lammps
165 lines
5.4 KiB
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165 lines
5.4 KiB
HTML
<HTML>
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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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<H3>units command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>units style
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</PRE>
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<UL><LI>style = <I>lj</I> or <I>real</I> or <I>metal</I> or <I>si</I> or <I>cgs</I> or <I>electron</I>
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>units metal
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units lj
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</PRE>
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<P><B>Description:</B>
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</P>
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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.
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</P>
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<P>For all units except <I>lj</I>, 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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</P>
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<P>For style <I>lj</I>, 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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</P>
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<UL><LI>mass = mass or m
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<LI>distance = sigma, where x* = x / sigma
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<LI>time = tau, where tau = t* = t (epsilon / m / sigma^2)^1/2
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<LI>energy = epsilon, where E* = E / epsilon
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<LI>velocity = sigma/tau, where v* = v tau / sigma
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<LI>force = epsilon/sigma, where f* = f sigma / epsilon
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<LI>torque = epsilon, where t* = t / epsilon
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<LI>temperature = reduced LJ temperature, where T* = T Kb / epsilon
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<LI>pressure = reduced LJ pressure, where P* = P sigma^3 / epsilon
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<LI>dynamic viscosity = reduced LJ viscosity, where eta* = eta sigma^3 / epsilon / tau
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<LI>charge = reduced LJ charge, where q* = q / (4 pi perm0 sigma epsilon)^1/2
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<LI>dipole = reduced LJ dipole, moment where *mu = mu / (4 pi perm0 sigma^3 epsilon)^1/2
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<LI>electric field = force/charge, where E* = E (4 pi perm0 sigma epsilon)^1/2 sigma / epsilon
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<LI>density = mass/volume, where rho* = rho sigma^dim
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</UL>
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<P>For style <I>real</I>, these are the units:
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</P>
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<UL><LI>mass = grams/mole
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<LI>distance = Angstroms
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<LI>time = femtoseconds
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<LI>energy = Kcal/mole
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<LI>velocity = Angstroms/femtosecond
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<LI>force = Kcal/mole-Angstrom
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<LI>torque = Kcal/mole
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<LI>temperature = degrees K
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<LI>pressure = atmospheres
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<LI>dynamic viscosity = Poise
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<LI>charge = multiple of electron charge (+1.0 is a proton)
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<LI>dipole = charge*Angstroms
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<LI>electric field = volts/Angstrom
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<LI>density = gram/cm^dim
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</UL>
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<P>For style <I>metal</I>, these are the units:
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</P>
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<UL><LI>mass = grams/mole
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<LI>distance = Angstroms
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<LI>time = picoseconds
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<LI>energy = eV
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<LI>velocity = Angstroms/picosecond
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<LI>force = eV/Angstrom
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<LI>torque = eV
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<LI>temperature = degrees K
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<LI>pressure = bars
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<LI>dynamic viscosity = Poise
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<LI>charge = multiple of electron charge (+1.0 is a proton)
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<LI>dipole = charge*Angstroms
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<LI>electric field = volts/Angstrom
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<LI>density = gram/cm^dim
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</UL>
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<P>For style <I>si</I>, these are the units:
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</P>
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<UL><LI>mass = kilograms
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<LI>distance = meters
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<LI>time = seconds
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<LI>energy = Joules
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<LI>velocity = meters/second
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<LI>force = Newtons
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<LI>torque = Newton-meters
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<LI>temperature = degrees K
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<LI>pressure = Pascals
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<LI>dynamic viscosity = Pascal*second
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<LI>charge = Coulombs
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<LI>dipole = Coulombs*meters
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<LI>electric field = volts/meter
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<LI>density = kilograms/meter^dim
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</UL>
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<P>For style <I>cgs</I>, these are the units:
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</P>
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<UL><LI>mass = grams
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<LI>distance = centimeters
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<LI>time = seconds
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<LI>energy = ergs
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<LI>velocity = centimeters/second
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<LI>force = dynes
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<LI>torque = dyne-centimeters
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<LI>temperature = degrees K
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<LI>pressure = dyne/cm^2 or barye = 1.0e-6 bars
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<LI>dynamic viscosity = Poise
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<LI>charge = statcoulombs or esu
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<LI>dipole = statcoul-cm = 10^18 debye
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<LI>electric field = statvolt/cm or dyne/esu
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<LI>density = grams/cm^dim
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</UL>
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<P>For style <I>electron</I>, these are the units:
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</P>
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<UL><LI>mass = atomic mass units
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<LI>distance = Bohr
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<LI>time = femtoseconds
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<LI>energy = Hartrees
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<LI>velocity = Bohr/atomic time units [1.03275e-15 seconds]
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<LI>force = Hartrees*Bohr
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<LI>temperature = degrees K
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<LI>pressure = Pascals
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<LI>charge = multiple of electron charge (+1.0 is a proton)
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<LI>dipole moment = Debye
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<LI>electric field = volts/cm
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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:
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</P>
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<UL><LI>For style <I>lj</I> these are dt = 0.005 tau and skin = 0.3 sigma.
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<LI>For style <I>real</I> these are dt = 1.0 fmsec and skin = 2.0 Angstroms.
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<LI>For style <I>metal</I> these are dt = 0.001 psec and skin = 2.0 Angstroms.
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<LI>For style <I>si</I> these are dt = 1.0e-8 sec and skin = 0.001 meters.
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<LI>For style <I>cgs</I> these are dt = 1.0e-8 sec and skin = 0.1 cm.
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<LI>For style <I>electron</I> these are dt = 0.001 fmsec and skin = 2.0 Bohr.
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</UL>
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<P><B>Restrictions:</B>
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</P>
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<P>This command cannot be used after the simulation box is defined by a
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<A HREF = "read_data.html">read_data</A> or <A HREF = "create_box.html">create_box</A> command.
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</P>
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<P><B>Related commands:</B> none
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</P>
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<P><B>Default:</B>
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</P>
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<PRE>units lj
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</PRE>
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