forked from lijiext/lammps
201 lines
8.7 KiB
Plaintext
201 lines
8.7 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 values ... :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 :l
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keyword = {sphere} or {cylinder} or {plane} or {side} 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 (distance units)
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R = sphere radius of indenter (distance units)
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any of x,y,z,R can be a variable (see below)
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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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any of c1,c2,R can be a variable (see below)
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{plane} args = dim pos side
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dim = {x} or {y} or {z} = 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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pos can be a variable (see below)
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side = {lo} or {hi}
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{side} value = {in} or {out}
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{in} = the indenter acts on particles inside the sphere or cylinder
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{out} = the indenter acts on particles outside the sphere or cylinder
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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
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fix 1 all indent 10.0 sphere v_x v_y 0.0 v_radius side in
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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. Or it can be used as a constraining wall
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around a simulation; see the discussion of the {side} keyword below.
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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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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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Spherical and cylindrical indenters account for periodic boundaries in
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two ways. First, the center point of a spherical indenter (x,y,z) or
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axis of a cylindrical indenter (c1,c2) is remapped back into the
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simulation box, if the box is periodic in a particular dimension.
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This occurs every timestep if the indenter geometry is specified with
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a variable (see below), e.g. it is moving over time. Second, the
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calculation of distance to the indenter center or axis accounts for
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periodic boundaries. Both of these mean that an indenter can
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effectively move through and straddle one or more periodic boundaries.
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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 {R} is the
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position of the plane and {r-R} is the distance from the plane. If
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the {side} parameter of the plane is specified as {lo} 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 {side}
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is specified as {hi}.
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Any of the 4 quantities defining a spherical indenter's geometry can
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be specified as an equal-style "variable"_variable.html, namely {x},
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{y}, {z}, or {R}. Similarly, for a cylindrical indenter, any of {c1},
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{c2}, or {R}, can be a variable. For a planar indenter, {pos} can be
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a variable. If the value is a variable, it should be specified as
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v_name, where name is the variable name. In this case, the variable
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will be evaluated each timestep, and its value used to define the
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indenter geometry.
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Note that equal-style variables can specify formulas with various
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mathematical functions, and include "thermo_style"_thermo_style.html
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command keywords for the simulation box parameters and timestep and
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elapsed time. Thus it is easy to specify indenter properties that
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change as a function of time or span consecutive runs in a continuous
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fashion. For the latter, see the {start} and {stop} keywords of the
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"run"_run.html command and the {elaplong} keyword of "thermo_style
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custom"_thermo_style.html for details.
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For example, if a spherical indenter's x-position is specfied as v_x,
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then this variable definition will keep it's center at a relative
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position in the simulation box, 1/4 of the way from the left edge to
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the right edge, even if the box size changes:
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variable x equal "xlo + 0.25*lx" :pre
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Similarly, either of these variable definitions will move the indenter
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from an initial position at 2.5 at a constant velocity of 5:
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variable x equal "2.5 + 5*elaplong*dt"
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variable x equal vdisplace(2.5,5) :pre
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If a spherical indenter's radius is specified as v_r, then these
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variable definitions will grow the size of the indenter at a specfied
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rate.
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variable r0 equal 0.0
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variable rate equal 1.0
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variable r equal "v_r0 + step*dt*v_rate" :pre
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If the {side} keyword is specified as {out}, 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 {side} keyword is specified as {in},
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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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The {units} keyword determines the meaning of the distance units used
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to define the indenter geometry. A {box} value selects standard
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distance units as defined by the "units"_units.html command,
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e.g. Angstroms for units = real or metal. A {lattice} value means the
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distance units are in lattice spacings. The "lattice"_lattice.html
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command must have been previously used to define the lattice spacing.
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The (x,y,z) coords of the indenter position are scaled by the x,y,z
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lattice spacings respectively. The radius of a spherical or
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cylindrical indenter is scaled by the x lattice spacing.
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Note that the units keyword only affects indenter geometry parameters
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specified directly with numbers, not those specified as variables. In
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the latter case, you should use the {xlat}, {ylat}, {zlat} keywords of
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the "thermo_style"_thermo_style.html command if you want to include
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lattice spacings in a variable formula.
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The force constant {K} is not affected by the {units} keyword. It is
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always in force/distance^2 units where force and distance are defined
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by the "units"_units.html command. If you wish K to be scaled by the
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lattice spacing, you can define K with a variable whose formula
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contains {xlat}, {ylat}, {zlat} keywords of the
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"thermo_style"_thermo_style.html command, e.g.
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variable k equal 100.0/xlat/xlat
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fix 1 all indent $k sphere ... :pre
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[Restart, fix_modify, output, run start/stop, minimize info:]
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No information about this fix is written to "binary restart
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files"_restart.html.
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The "fix_modify"_fix_modify.html {energy} 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 "thermodynamic
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output"_thermo_style.html. The energy of each particle interacting
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with the indenter is K/3 (r - R)^3.
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This fix computes a global scalar energy and a global 3-vector of
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forces (on the indenter), which can be accessed by various "output
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commands"_Section_howto.html#4_15. The scalar and vector values
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calculated by this fix are "extensive".
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The forces due to this fix are imposed during an energy minimization,
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invoked by the "minimize"_minimize.html command. Note that if you
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define the indenter geometry with a variable using a time-dependent
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formula, LAMMPS uses the iteration count in the minimizer as the
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timestep. But it is almost certainly a bad idea to have the indenter
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change its position or size during a minimization. LAMMPS does not
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check if you have done this.
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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 "fix_modify"_fix_modify.html
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{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 side = out and units = lattice.
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