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2010-09-16 19:46:13 +00:00
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<HTML>
<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>
</CENTER>
<HR>
<H3>fix external command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>fix ID group-ID external
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
<LI>external = style name of this fix command
</UL>
<P><B>Examples:</B>
</P>
<PRE>fix 1 all external
</PRE>
<P><B>Description:</B>
</P>
<P>This fix makes a callback each timestep to an external driver program
that is using LAMMPS as a library. This is a way to let another
program compute forces on atoms which LAMMPS will include in its
dynamics performed by the <A HREF = "run.html">run</A> command.
</P>
<P>The callback function "foo" will be invoked every timestep as:
</P>
<PRE>foo(ptr,timestep,nlocal,ids,x,fexternal);
</PRE>
<P>which has this prototype:
</P>
<P>void foo(void *, int, int, int *, double **, double **);
</P>
<P>The arguments are as follows:
</P>
<UL><LI>ptr = pointer provided by and simply passed back to external driver
<LI>timestep = current LAMMPS timestep
<LI>nlocal = # of atoms on this processor
<LI>ids = list of atom IDs on this processor
<LI>x = coordinates of atoms on this processor
<LI>fexternal = forces on atoms on this processor
</UL>
<P>Fexternal are the forces returned by the driver program,
which LAMMPS adds to the current force on each atom.
</P>
<P>See the couple/lammps_quest/lmpqst.cpp file in the LAMMPS distribution
for an example of a coupling application that uses this fix, and how
it makes a call to the fix to specify what function the fix should
callback to. The sample application performs classical MD using
quantum forces computed by a density functional code <A HREF = "http://dft.sandia.gov/Quest">Quest</A>.
</P>
<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
</P>
<P>No information about this fix is written to <A HREF = "restart.html">binary restart
files</A>. None of the <A HREF = "fix_modify.html">fix_modify</A> options
are relevant to this fix. No global or per-atom quantities are stored
by this fix for access by various <A HREF = "Section_howto.html#4_15">output
commands</A>. No parameter of this fix can be
used with the <I>start/stop</I> keywords of the <A HREF = "run.html">run</A> command.
This fix is not invoked during <A HREF = "minimize.html">energy minimization</A>.
</P>
<P><B>Restrictions:</B> none
</P>
<P><B>Related commands:</B> none
</P>
<P><B>Default:</B> none
</P>
</HTML>