forked from lijiext/lammps
113 lines
4.5 KiB
Plaintext
113 lines
4.5 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Section_commands.html#comm)
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:line
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fix indent command :h3
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[Syntax:]
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fix ID group-ID indent k keyword args ... :pre
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ID, group-ID are documented in "fix"_fix.html command :ulb,l
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indent = style name of this fix command :l
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k = force constant for indenter surface (force/distance^2 units) :l
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one or more keyword/value pairs may be appended to the args :l
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keyword = {sphere} or {cylinder} or {vel} or {rstart} or {units} :l
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{sphere} 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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{cylinder} 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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{vel} args = vx vy vz
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vx,vy,vz = velocity of center of indenter (velocity units)
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{rstart} value = R0
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R0 = sphere or cylinder radius at start of run
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R is value at end of run, so indenter expands/contracts over time
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{units} value = {lattice} or {box}
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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 :pre
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:ule
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[Examples:]
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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 :pre
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[Description:]
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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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The indenter can either be spherical or cylindrical. You must set
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one of those 2 keywords.
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A spherical indenter exerts a force of magnitude
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F(r) = - k (r - R)^2 :pre
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on each atom where {k} is the specified force constant, {r} is the
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distance from the atom to the center of the indenter, and {R} is the
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radius of the indenter. The force is repulsive and F(r) = 0 for {r} >
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{R}.
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A cylindrical indenter exerts the same force, except that {r} 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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If the {vel} 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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If the {rstart} 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. The
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"run"_run.html command documents how to make the interpolation take
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place across multiple runs. This option can be used, for example, to
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grow/shrink a void within the simulation box. This option is not
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relevant during an energy minimization; the indenter always has radius
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R in that case. Note that if you do multiple runs, you may need to
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re-specify the fix so that the indenter radius has the appropriate
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value. If you do nothing, it will be reset to R0 at the beginning of
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each run.
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The {units} keyword determines the meaning of the distance units used
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to define the indenter. A {box} value selects standard distance units
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as defined by the "units"_units.html command, e.g. Angstroms for units
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= real or metal. A {lattice} value means the distance units are in
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cubic lattice spacings. The "lattice"_lattice.html command must first
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be used to define a lattice. Note that the units choice affects not
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only the indenter's physical geometry, but also its velocity and force
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constant since they are defined in terms of distance as well.
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This fix supports the "fix_modify"_fix_modify.html options for
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{thermo} and {energy}. The former will print the contribution the fix
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makes to the energy of the system when thermodynamics is printed,
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where the energy of each particle interacting with the indenter is K/3
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(r - R)^3. The latter will add this contribution to the total
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potential energy (PotEng).
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The forces due to this fix are also imposed during an energy
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minimization, invoked by the "minimize"_minimize.html command. If you
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want that energy to be included in the total potential energy of the
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system (the quantity being minimized), you must enable the
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"fix_modify"_fix_modify.html {energy} option for this fix.
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[Restrictions:] none
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[Related commands:] none
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[Default:]
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The option defaults are vel = 0,0,0 and units = lattice.
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