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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>fix aveforce command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>fix ID group-ID aveforce fx fy fz
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
<LI>aveforce = style name of this fix command
<LI>fx,fy,fz = force component values (force units)
</UL>
<P><B>Examples:</B>
</P>
<PRE>fix pressdown topwall aveforce 0.0 -1.0 0.0
fix 2 bottomwall aveforce NULL -1.0 0.0
</PRE>
<P><B>Description:</B>
</P>
<P>Apply an additional external force to a group of atoms in such a way
that every atom experiences the same force. This is useful for
pushing on wall or boundary atoms so that the structure of the wall
does not change over time.
</P>
<P>The existing force is averaged for the group of atoms, component by
component. The actual force on each atom is then set to the average
value plus the component specified in this command. This means each
atom in the group receives the same force.
</P>
<P>If any of the arguments is specified as NULL then the forces in that
dimension are not changed. Note that this is not the same as
specifying a 0.0 value, since that sets all forces to the same average
value without adding in any additional force.
</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.
</P>
<P>This fix computes a 3-vector of forces, which can be accessed by
various <A HREF = "Section_howto.html#4_15">output commands</A>. This is the total
force on the group of atoms before the forces on individual atoms are
changed by the fix. The vector values calculated by this fix are
"extensive", meaning they scale with the number of atoms in the
simulation.
</P>
<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
the <A HREF = "run.html">run</A> command.
</P>
<P>The forces due to this fix are imposed during an energy minimization,
invoked by the <A HREF = "minimize.html">minimize</A> command.
</P>
<P><B>Restrictions:</B> none
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "fix_setforce.html">fix setforce</A>, <A HREF = "fix_addforce.html">fix addforce</A>
</P>
<P><B>Default:</B> none
</P>
</HTML>
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