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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 indent command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>fix ID group-ID indent k keyword args ...
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
<LI>indent = style name of this fix command
<LI>k = force constant for indenter surface (force/distance^2 units)
<LI>one or more keyword/value pairs may be appended to the args
<LI>keyword = <I>sphere</I> or <I>cylinder</I> or <I>vel</I> or <I>rstart</I> or <I>units</I>
<PRE> <I>sphere</I> args = x y z R
x,y,z = initial position of center of indenter
R = sphere radius of indenter (distance units)
<I>cylinder</I> args = dim c1 c2 R
dim = x or y or z = axis of cylinder
c1,c2 = coords of cylinder axis in other 2 dimensions (distance units)
R = cylinder radius of indenter (distance units)
<I>vel</I> args = vx vy vz
vx,vy,vz = velocity of center of indenter (velocity units)
<I>rstart</I> value = R0
R0 = sphere or cylinder radius at start of run
R is value at end of run, so indenter expands/contracts over time
<I>units</I> value = <I>lattice</I> or <I>box</I>
lattice = the geometry is defined in lattice units
box = the geometry is defined in simulation box units
</PRE>
</UL>
<P><B>Examples:</B>
</P>
<PRE>fix 1 all indent 10.0 sphere 0.0 0.0 15.0 3.0 vel 0.0 0.0 -1.0
fix 2 flow indent 10.0 cylinder z 0.0 0.0 10.0 units box
</PRE>
<P><B>Description:</B>
</P>
<P>Insert an indenter within a simulation box. The indenter repels all
atoms that touch it, so it can be used to push into a material or as
an obstacle in a flow.
</P>
<P>The indenter can either be spherical or cylindrical. You must set
one of those 2 keywords.
</P>
<P>A spherical indenter exerts a force of magnitude
</P>
<PRE>F(r) = - k (r - R)^2
</PRE>
<P>on each atom where <I>k</I> is the specified force constant, <I>r</I> is the
distance from the atom to the center of the indenter, and <I>R</I> is the
radius of the indenter. The force is repulsive and F(r) = 0 for <I>r</I> >
<I>R</I>.
</P>
<P>A cylindrical indenter exerts the same force, except that <I>r</I> is the
distance from the atom to the center axis of the cylinder. The
cylinder extends infinitely along its axis.
</P>
<P>If the <I>vel</I> keyword is specified, the center (or axis) of the
spherical (or cylindrical) indenter will move during the simulation,
based on its initial position (x,y,z) and the specified (vx,vy,vz).
Note that if you do multiple runs, the initial position of the
indenter (x,y,z) does not change, so it will continue to move at the
specified velocity.
</P>
<P>If the <I>rstart</I> keyword is specified, then the radius of the indenter
is a time-dependent quantity. R0 is the value assigned at the start
of the run; R is the value at the end. At intermediate times, the
radius is linearly interpolated between these two values. The
<A HREF = "run.html">run</A> command documents how to make the interpolation take
place across multiple runs. This option can be used, for example, to
grow/shrink a void within the simulation box. This option is not
relevant during an energy minimization; the indenter always has radius
R in that case. Note that if you do multiple runs, you may need to
re-specify the fix so that the indenter radius has the appropriate
value. If you do nothing, it will be reset to R0 at the beginning of
each run.
</P>
<P>The <I>units</I> keyword determines the meaning of the distance units used
to define the indenter. A <I>box</I> value selects standard distance units
as defined by the <A HREF = "units.html">units</A> command, e.g. Angstroms for units
= real or metal. A <I>lattice</I> value means the distance units are in
cubic lattice spacings. The <A HREF = "lattice.html">lattice</A> command must first
be used to define a lattice. Note that the units choice affects not
only the indenter's physical geometry, but also its velocity and force
constant since they are defined in terms of distance as well.
</P>
<P>This fix supports the <A HREF = "fix_modify.html">fix_modify</A> options for
<I>thermo</I> and <I>energy</I>. The former will print the contribution the fix
makes to the energy of the system when thermodynamics is printed,
where the energy of each particle interacting with the indenter is K/3
(r - R)^3. The latter will add this contribution to the total
potential energy (PotEng).
</P>
<P>The forces due to this fix are also imposed during an energy
minimization, invoked by the <A HREF = "minimize.html">minimize</A> command. If you
want that energy 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> none
</P>
<P><B>Default:</B>
</P>
<P>The option defaults are vel = 0,0,0 and units = lattice.
</P>
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