forked from lijiext/lammps
82 lines
4.1 KiB
Plaintext
82 lines
4.1 KiB
Plaintext
"Higher level section"_Howto.html - "LAMMPS WWW Site"_lws - "LAMMPS
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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,Commands_all.html)
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:line
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Walls :h3
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Walls in an MD simulation are typically used to bound particle motion,
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i.e. to serve as a boundary condition.
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Walls in LAMMPS can be of rough (made of particles) or idealized
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surfaces. Ideal walls can be smooth, generating forces only in the
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normal direction, or frictional, generating forces also in the
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tangential direction.
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Rough walls, built of particles, can be created in various ways. The
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particles themselves can be generated like any other particle, via the
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"lattice"_lattice.html and "create_atoms"_create_atoms.html commands,
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or read in via the "read_data"_read_data.html command.
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Their motion can be constrained by many different commands, so that
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they do not move at all, move together as a group at constant velocity
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or in response to a net force acting on them, move in a prescribed
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fashion (e.g. rotate around a point), etc. Note that if a time
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integration fix like "fix nve"_fix_nve.html or "fix nvt"_fix_nh.html
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is not used with the group that contains wall particles, their
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positions and velocities will not be updated.
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"fix aveforce"_fix_aveforce.html - set force on particles to average value, so they move together
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"fix setforce"_fix_setforce.html - set force on particles to a value, e.g. 0.0
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"fix freeze"_fix_freeze.html - freeze particles for use as granular walls
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"fix nve/noforce"_fix_nve_noforce.html - advect particles by their velocity, but without force
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"fix move"_fix_move.html - prescribe motion of particles by a linear velocity, oscillation, rotation, variable :ul
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The "fix move"_fix_move.html command offers the most generality, since
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the motion of individual particles can be specified with
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"variable"_variable.html formula which depends on time and/or the
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particle position.
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For rough walls, it may be useful to turn off pairwise interactions
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between wall particles via the "neigh_modify
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exclude"_neigh_modify.html command.
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Rough walls can also be created by specifying frozen particles that do
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not move and do not interact with mobile particles, and then tethering
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other particles to the fixed particles, via a "bond"_bond_style.html.
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The bonded particles do interact with other mobile particles.
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Idealized walls can be specified via several fix commands. "Fix
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wall/gran"_fix_wall_gran.html creates frictional walls for use with
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granular particles; all the other commands create smooth walls.
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"fix wall/reflect"_fix_wall_reflect.html - reflective flat walls
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"fix wall/lj93"_fix_wall.html - flat walls, with Lennard-Jones 9/3 potential
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"fix wall/lj126"_fix_wall.html - flat walls, with Lennard-Jones 12/6 potential
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"fix wall/colloid"_fix_wall.html - flat walls, with "pair_style colloid"_pair_colloid.html potential
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"fix wall/harmonic"_fix_wall.html - flat walls, with repulsive harmonic spring potential
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"fix wall/morse"_fix_wall.html - flat walls, with Morse potential
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"fix wall/region"_fix_wall_region.html - use region surface as wall
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"fix wall/gran"_fix_wall_gran.html - flat or curved walls with "pair_style granular"_pair_gran.html potential :ul
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The {lj93}, {lj126}, {colloid}, {harmonic}, and {morse} styles all
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allow the flat walls to move with a constant velocity, or oscillate in
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time. The "fix wall/region"_fix_wall_region.html command offers the
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most generality, since the region surface is treated as a wall, and
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the geometry of the region can be a simple primitive volume (e.g. a
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sphere, or cube, or plane), or a complex volume made from the union
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and intersection of primitive volumes. "Regions"_region.html can also
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specify a volume "interior" or "exterior" to the specified primitive
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shape or {union} or {intersection}. "Regions"_region.html can also be
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"dynamic" meaning they move with constant velocity, oscillate, or
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rotate.
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The only frictional idealized walls currently in LAMMPS are flat or
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curved surfaces specified by the "fix wall/gran"_fix_wall_gran.html
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command. At some point we plan to allow regoin surfaces to be used as
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frictional walls, as well as triangulated surfaces.
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