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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>
</CENTER>
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<H3>fix addforce command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>fix ID group-ID addforce fx fy fz
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
<LI>addforce = style name of this fix command
<LI>fx,fy,fz = force component values (force units)
</UL>
<P><B>Examples:</B>
</P>
<PRE>fix kick flow addforce 1.0 0.0 0.0
</PRE>
<P><B>Description:</B>
</P>
<P>Add fx,fy,fz to the corresponding component of force for each atom in
the group. This command can be used to give an additional push to
atoms in a simulation, such as for a simulation of Poiseuille flow in
a channel.
</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>.
</P>
<P>The <A HREF = "fix_modify.html">fix_modify</A> <I>energy</I> option is supported by this
fix to add the potential "energy" inferred by the added force to the
system's potential energy as part of <A HREF = "thermo_style.html">thermodynamic
output</A>. This is a fictitious quantity but is
needed so that the <A HREF = "minimize.html">minimize</A> command can include the
forces added by this fix in a consistent manner. I.e. there is a
decrease in potential energy when atoms move in the direction of the
added force.
</P>
<P>This fix computes a scalar and a 3-vector of forces, which can be
accessed by various <A HREF = "Section_howto.html#4_15">output commands</A>. The
scalar is the potential energy discussed above. The vector is the
total force on the group of atoms before the forces on individual
atoms are changed by the fix. The scalar 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>IMPORTANT NOTE: If you want the fictitious potential energy associated
with the added forces to be included in the total potential energy of
the system (the quantity being minimized), you MUST enable the
<A HREF = "fix_modify.html">fix_modify</A> <I>energy</I> option for this fix.
</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_aveforce.html">fix aveforce</A>
</P>
<P><B>Default:</B> none
</P>
</HTML>
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