forked from lijiext/lammps
122 lines
5.0 KiB
Plaintext
122 lines
5.0 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 wall/reflect command :h3
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[Syntax:]
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fix ID group-ID wall/reflect face arg ... keyword value ... :pre
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ID, group-ID are documented in "fix"_fix.html command :ulb,l
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wall/reflect = style name of this fix command :l
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one or more face/arg pairs may be appended :l
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face = {xlo} or {xhi} or {ylo} or {yhi} or {zlo} or {zhi} :l
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{xlo},{ylo},{zlo} arg = EDGE or constant or variable
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EDGE = current lo edge of simulation box
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constant = number like 0.0 or -30.0 (distance units)
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variable = "equal-style variable"_variable.html like v_x or v_wiggle
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{xhi},{yhi},{zhi} arg = EDGE or constant or variable
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EDGE = current hi edge of simulation box
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constant = number like 50.0 or 100.3 (distance units)
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variable = "equal-style variable"_variable.html like v_x or v_wiggle :pre
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zero or more keyword/avlue pairs may be appended :l
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keyword = {units} :l
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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 xwalls all wall/reflect xlo EDGE xhi EDGE
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fix walls all wall/reflect xlo 0.0 ylo 10.0 units box
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fix top all wall/reflect zhi v_pressdown :pre
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[Description:]
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Bound the simulation with one or more walls which reflect particles
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in the specified group when they attempt to move thru them.
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Reflection means that if an atom moves outside the wall on a timestep
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by a distance delta (e.g. due to "fix nve"_fix_nve.html), then it is
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put back inside the face by the same delta, and the sign of the
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corresponding component of its velocity is flipped.
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When used in conjunction with "fix nve"_fix_nve.html and "run_style
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verlet"_run_style.html, the resultant time-integration algorithm is
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equivalent to the primitive splitting algorithm (PSA) described by
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"Bond"_#Bond. Because each reflection event divides
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the corresponding timestep asymmetrically, energy conservation is only
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satisfied to O(dt), rather than to O(dt^2) as it would be for
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velocity-Verlet integration without reflective walls.
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Up to 6 walls or faces can be specified in a single command: {xlo},
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{xhi}, {ylo}, {yhi}, {zlo}, {zhi}. A {lo} face reflects particles
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that move to a coordinate less than the face position, back in the
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{hi} direction. A {hi} face reflects particles that move to a
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coordinate higher than the face position, back in the {lo} direction.
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The position of each face can be specified in one of 3 ways: as the
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EDGE of the simulation box, as a constant value, or as a variable. If
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EDGE is used, then the corresponding boundary of the current
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simulation box is used. If a numeric constant is specified then the
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wall is placed at that position in the appropriate dimension (x, y, or
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z). In both the EDGE and constant cases, the wall will never move.
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If the face position is a variable, it should be specified as v_name,
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where name is an "equal-style variable"_variable.html name. In this
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case the variable is evaluated each timestep and the result becomes
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the current position of the reflecting wall. Equal-style variables
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can specify formulas with various mathematical functions, and include
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"thermo_style"_thermo_style.html command keywords for the simulation
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box parameters and timestep and elapsed time. Thus it is easy to
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specify a time-dependent wall position.
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The {units} keyword determines the meaning of the distance units used
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to define a face position, but only when a numeric constant is used.
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It is not relevant when EDGE or a variable is used to specify a face
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position.
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A {box} value selects standard distance units as defined by the
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"units"_units.html command, e.g. Angstroms for units = real or metal.
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A {lattice} value means the distance units are in lattice spacings.
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The "lattice"_lattice.html command must have been previously used to
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define the lattice spacings.
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:line
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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. None of the "fix_modify"_fix_modify.html options
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are relevant to this fix. No global or per-atom quantities are stored
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by this fix for access by various "output
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commands"_Section_howto.html#4_15. No parameter of this fix can be
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used with the {start/stop} keywords of the "run"_run.html command.
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This fix is not invoked during "energy minimization"_minimize.html.
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[Restrictions:]
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Any dimension (xyz) that has a reflecting wall must be non-periodic.
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A reflecting wall should not be used with rigid bodies such as those
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defined by a "fix rigid" command. This is because the wall/reflect
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displaces atoms directly rather than exerts a force on them. For
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rigid bodies, use a soft wall instead, such as "fix
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wall/lj93"_fix_wall.html. LAMMPS will flag the use of a rigid
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fix with fix wall/reflect with a warning, but will not generate an
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error.
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[Related commands:]
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"fix wall/lj93"_fix_wall.html command
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[Default:] none
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:link(Bond)
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[(Bond)] Bond and Leimkuhler, SIAM J Sci Comput, 30, p 134 (2007).
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