forked from lijiext/lammps
176 lines
8.0 KiB
HTML
176 lines
8.0 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>plane</I> or <I>vel</I> or <I>rstart</I> or <I>side</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 (distance units)
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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 = <I>x</I> or <I>y</I> or <I>z</I> = 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>plane</I> args = dim pos side
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dim = <I>x</I> or <I>y</I> or <I>z</I> = plane perpendicular to this dimension
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pos = position of plane in dimension x, y, or z (distance units)
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side = <I>lo</I> or <I>hi</I>
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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>side</I> value = <I>in</I> or <I>out</I>
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<I>in</I> = the indenter acts on particles inside the sphere or cylinder
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<I>out</I> = the indenter acts on particles outside the sphere or cylinder
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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. Or it can be used as a constraining wall
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around a simulation; see the discussion of the <I>side</I> keyword below.
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</P>
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<P>The indenter can either be spherical or cylindrical or planar. You
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must set one of those 3 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>. The calculation of distance to the indenter center accounts
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for periodic boundaries, which means the indenter can effectively
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straddle one or more periodic boundaries.
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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. The calculation of
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distance to the indenter axis accounts for periodic boundaries, which
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means the indenter can effectively straddle one or more periodic
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boundaries.
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</P>
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<P>A planar indenter is really an axis-aligned infinite-extent wall
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exerting the same force on atoms in the system, where <I>R</I> is the
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position of the plane and <I>r-R</I> is the distance from the plane. If
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the <I>side</I> parameter of the plane is specified as <I>lo</I> then it will
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indent from the lo end of the simulation box, meaning that atoms with
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a coordinate less than the plane's current position will be pushed
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towards the hi end of the box and atoms with a coordinate higher than
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the plane's current position will feel no force. Vice versa if <I>side</I>
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is specified as <I>hi</I>.
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</P>
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<P>If the <I>vel</I> keyword is specified, the center (or axis or position) of
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the spherical (or cylindrical or planar) indenter will move during the
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simulation, based on its initial position (x,y,z), the specified
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(vx,vy,vz), and the time elapsed since the beginning of the
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simulation. For periodic systems and spherical or cylindrical
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indenters, the new position of the center or axis is wrapped back into
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the periodic box as needed. See the note below about making the
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indenter move continuously across multiple runs.
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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. This only applies to spherical or
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cylindrical indenters. R0 is the value assigned at the start of the
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run; R is the value at the end. At intermediate times, the radius is
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linearly interpolated between these two values. This option can be
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used, for example, to grow/shrink a void within the simulation box.
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See the note below about making the radius change continuously across
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multiple runs.
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</P>
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<P>If the <I>side</I> keyword is specified as <I>out</I>, which is the default,
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then particles outside the indenter are pushded away from its outer
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surface, as described above. This only applies to spherical or
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cylindrical indenters. If the <I>side</I> keyword is specified as <I>in</I>,
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the action of the indenter is reversed. Particles inside the indenter
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are pushed away from its inner surface. In other words, the indenter
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is now a containing wall that traps the particles inside it. If the
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radius shrinks over time, it will squeeze the particles.
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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><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. If you do not do this, the indenter position and readius will
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be reset to their specified initial values at the beginning of each
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run.
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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, side = out, and units = lattice.
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</P>
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</HTML>
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