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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
</CENTER>
<HR>
<H3>units command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>units style
</PRE>
<UL><LI>style = <I>lj</I> or <I>real</I> or <I>metal</I>
</UL>
<P><B>Examples:</B>
</P>
<PRE>units metal
units lj
</PRE>
<P><B>Description:</B>
</P>
<P>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.
</P>
<P>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.
</P>
<P>For style <I>lj</I>, all quantities are unitless:
</P>
<UL><LI>distance = sigma
<LI>time = tau
<LI>mass = one
<LI>energy = epsilon
<LI>velocity = sigma/tau
<LI>force = epsilon/sigma
<LI>temperature = reduced LJ temperature
<LI>pressure = reduced LJ pressure
<LI>charge = reduced LJ charge
<LI>dipole = reduced LJ dipole moment
<LI>electric field = force/charge
</UL>
<P>For style <I>real</I>, these are the units:
</P>
<UL><LI>distance = Angstroms
<LI>time = femtoseconds
<LI>mass = grams/mole
<LI>energy = Kcal/mole
<LI>velocity = Angstroms/femtosecond
<LI>force = Kcal/mole-Angstrom
<LI>temperature = degrees K
<LI>pressure = atmospheres
<LI>charge = multiple of electron charge (+1.0 is a proton)
<LI>dipole = charge*Angstroms
<LI>electric field = volts/Angstrom
</UL>
<P>For style <I>metal</I>, these are the units:
</P>
<UL><LI>distance = Angstroms
<LI>time = picoseconds
<LI>mass = grams/mole
<LI>energy = eV
<LI>velocity = Angstroms/picosecond
<LI>force = eV/Angstrom
<LI>temperature = degrees K
<LI>pressure = bars
<LI>charge = multiple of electron charge (+1.0 is a proton)
<LI>dipole = charge*Angstroms
<LI>electric field = volts/Angstrom
</UL>
<P>This command also sets the timestep size and neighbor skin distance to
default values for each style. For style <I>lj</I> these are dt = 0.005
tau and skin = 0.3 sigma. For style <I>real</I> these are dt = 1.0 fmsec
and skin = 2.0 Angstroms. For style <I>metal</I> these are dt = 0.001 psec
and skin = 2.0 Angstroms.
</P>
<P><B>Restrictions:</B>
</P>
<P>This command cannot be used after the simulation box is defined by a
<A HREF = "read_data.html">read_data</A> or <A HREF = "create_box.html">create_box</A> command.
</P>
<P><B>Related commands:</B> none
</P>
<P><B>Default:</B>
</P>
<PRE>units lj
</PRE>
</HTML>
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