forked from lijiext/lammps
94 lines
2.5 KiB
HTML
94 lines
2.5 KiB
HTML
<HTML>
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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>
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<HR>
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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>
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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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detemines 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 real and metallic units, LAMMPS uses physical constants from
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www.physics.nist.gov. For the definition of Kcal in real units, LAMMPS
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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:
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</P>
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<UL><LI>distance = sigma
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<LI>time = tau
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<LI>mass = one
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<LI>energy = epsilon
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<LI>velocity = sigma/tau
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<LI>force = epsilon/sigma
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<LI>temperature = reduced LJ temperature
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<LI>pressure = reduced LJ pressure
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<LI>charge = reduced LJ charge
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<LI>electric field = force/charge
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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>distance = Angstroms
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<LI>time = femtoseconds
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<LI>mass = grams/mole
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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>temperature = degrees K
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<LI>pressure = atmospheres
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<LI>charge = multiple of electron charge (+1.0 is a proton)
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<LI>electric field = volts/Angstrom
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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>distance = Angstroms
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<LI>time = picoseconds
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<LI>mass = grams/mole
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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>temperature = degrees K
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<LI>pressure = bars
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<LI>charge = multiple of electron charge (+1.0 is a proton)
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<LI>electric field = volts/Angstrom
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</UL>
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<P>This command also sets the timestep size and neighbor skin distance to
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default values for each style. For style <I>lj</I> these are dt = 0.005
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tau and skin = 0.3 sigma. For style <I>real</I> these are dt = 1.0 fmsec
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and skin = 2.0 Angstroms. For style <I>metal</I> these are dt = 0.001 psec
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and skin = 2.0 Angstroms.
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</P>
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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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</HTML>
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