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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>
<HR>
<H3>fix drag command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>fix ID group-ID drag x y z fmag delta
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
<LI>drag = style name of this fix command
<LI>x,y,z = coord to drag atoms towards
<LI>fmag = magnitude of force to apply to each atom (force units)
<LI>delta = cutoff distance inside of which force is not applied (distance units)
</UL>
<P><B>Examples:</B>
</P>
<PRE>fix center small-molecule drag 0.0 10.0 0.0 5.0 2.0
</PRE>
<P><B>Description:</B>
</P>
<P>Apply a force to each atom in a group to drag it towards the point
(x,y,z). The magnitude of the force is specified by fmag. If an atom
is closer than a distance delta to the point, then the force is not
applied.
</P>
<P>Any of the x,y,z values can be specified as NULL which means do not
include that dimension in the distance calculation or force
application.
</P>
<P>This command can be used to steer one or more atoms to a new location
in the simulation.
</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 by the drag force. 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. 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>
</P>
<P><A HREF = "fix_spring.html">fix spring</A>, <A HREF = "fix_spring_self.html">fix spring/self</A>,
<A HREF = "fix_spring_rg.html">fix spring/rg</A>, <A HREF = "fix_smd.html">fix smd</A>
</P>
<P><B>Default:</B> none
</P>
</HTML>
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