forked from lijiext/lammps
142 lines
5.7 KiB
HTML
142 lines
5.7 KiB
HTML
<HTML>
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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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</CENTER>
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<HR>
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<H3>fix indent command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>fix ID group-ID indent k keyword values ...
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
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<LI>indent = style name of this fix command
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<LI>k = force constant for indenter surface (force/distance^2 units)
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<LI>one or more keyword/value pairs may be appended
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<LI>keyword = <I>sphere</I> or <I>cylinder</I> or <I>vel</I> or <I>rstart</I> or <I>units</I>
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<PRE> <I>sphere</I> args = x y z R
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x,y,z = initial position of center of indenter
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R = sphere radius of indenter (distance units)
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<I>cylinder</I> args = dim c1 c2 R
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dim = x or y or z = axis of cylinder
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c1,c2 = coords of cylinder axis in other 2 dimensions (distance units)
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R = cylinder radius of indenter (distance units)
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<I>vel</I> args = vx vy vz
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vx,vy,vz = velocity of center of indenter (velocity units)
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<I>rstart</I> value = R0
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R0 = sphere or cylinder radius at start of run (distance units)
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R is value at end of run, so indenter expands/contracts over time
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<I>units</I> value = <I>lattice</I> or <I>box</I>
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lattice = the geometry is defined in lattice units
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box = the geometry is defined in simulation box units
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</PRE>
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>fix 1 all indent 10.0 sphere 0.0 0.0 15.0 3.0 vel 0.0 0.0 -1.0
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fix 2 flow indent 10.0 cylinder z 0.0 0.0 10.0 units box
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Insert an indenter within a simulation box. The indenter repels all
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atoms that touch it, so it can be used to push into a material or as
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an obstacle in a flow.
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</P>
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<P>The indenter can either be spherical or cylindrical. You must set
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one of those 2 keywords.
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</P>
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<P>A spherical indenter exerts a force of magnitude
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</P>
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<PRE>F(r) = - k (r - R)^2
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</PRE>
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<P>on each atom where <I>k</I> is the specified force constant, <I>r</I> is the
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distance from the atom to the center of the indenter, and <I>R</I> is the
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radius of the indenter. The force is repulsive and F(r) = 0 for <I>r</I> >
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<I>R</I>.
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</P>
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<P>A cylindrical indenter exerts the same force, except that <I>r</I> is the
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distance from the atom to the center axis of the cylinder. The
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cylinder extends infinitely along its axis.
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</P>
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<P>If the <I>vel</I> keyword is specified, the center (or axis) of the
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spherical (or cylindrical) indenter will move during the simulation,
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based on its initial position (x,y,z) and the specified (vx,vy,vz).
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Note that if you do multiple runs, the initial position of the
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indenter (x,y,z) does not change, so it will continue to move at the
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specified velocity.
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</P>
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<P>If the <I>rstart</I> keyword is specified, then the radius of the indenter
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is a time-dependent quantity. R0 is the value assigned at the start
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of the run; R is the value at the end. At intermediate times, the
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radius is linearly interpolated between these two values. This option
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can be used, for example, to grow/shrink a void within the simulation
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box.
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</P>
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<P>The <I>units</I> keyword determines the meaning of the distance units used
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to define the indenter. A <I>box</I> value selects standard distance units
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as defined by the <A HREF = "units.html">units</A> command, e.g. Angstroms for units
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= real or metal. A <I>lattice</I> value means the distance units are in
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lattice spacings. The <A HREF = "lattice.html">lattice</A> command must have been
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previously used to define the lattice spacing. Note that the units
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choice affects not only the indenter's physical geometry, but also its
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velocity and force constant since they are defined in terms of
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distance as well.
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</P>
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<P>IMPORTANT NOTE: You should insure the indenter's extent does not
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overlap a periodic boundary, either for a fixed indenter, or one that
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moves. No check for such overlaps is performed by the code.
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</P>
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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</P>
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<P>No information about this fix is written to <A HREF = "restart.html">binary restart
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files</A>.
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</P>
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<P>The <A HREF = "fix_modify.html">fix_modify</A> <I>energy</I> option is supported by this
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fix to add the energy of interaction between atoms and the indenter to
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the system's potential energy as part of <A HREF = "thermo_style.html">thermodynamic
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output</A>. The energy of each particle interacting
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with the indenter is K/3 (r - R)^3.
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</P>
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<P>This fix computes a scalar energy and a 3-vector of forces (on the
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indenter), which can be accessed by various <A HREF = "Section_howto.html#4_15">output
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commands</A>. The scalar and vector values
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calculated by this fix are "extensive", meaning they scale with the
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number of atoms in the simulation.
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</P>
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<P>This fix can adjust the indenter position and radius over multiple
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runs, using the <I>start</I> and <I>stop</I> keywords of the <A HREF = "run.html">run</A>
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command. See the <A HREF = "run.html">run</A> command for details of how to do
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this.
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</P>
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<P>The forces due to this fix are imposed during an energy minimization,
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invoked by the <A HREF = "minimize.html">minimize</A> command. The <I>rstart</I> keyword
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does not change the indenter radius during an energy minimization; the
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indenter always has a radius of its final value R in that case.
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</P>
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<P>IMPORTANT NOTE: If you want the atom/indenter interaction energy to be
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included in the total potential energy of the system (the quantity
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being minimized), you must enable the <A HREF = "fix_modify.html">fix_modify</A>
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<I>energy</I> option for this fix.
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</P>
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<P><B>Restrictions:</B> none
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</P>
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<P><B>Related commands:</B> none
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</P>
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<P><B>Default:</B>
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</P>
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<P>The option defaults are vel = 0,0,0 and units = lattice.
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</P>
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</HTML>
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