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git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@3647 f3b2605a-c512-4ea7-a41b-209d697bcdaa

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@ -26,7 +26,8 @@ certain kinds of LAMMPS simulations.
4.13 "NEMD simulations"_#4_13
4.14 "Extended spherical and aspherical particles"_#4_14
4.15 "Output from LAMMPS (thermo, dumps, computes, fixes, variables)"_#4_15
4.16 "Thermostatting, barostatting and computing temperature"_#4_16 :all(b)
4.16 "Thermostatting, barostatting and computing temperature"_#4_16
4.17 "Walls"_#4_17 :all(b)
The example input scripts included in the LAMMPS distribution and
highlighted in "this section"_Section_example.html also show how to
@ -1348,6 +1349,78 @@ you can use the "thermo_modify"_thermo_modify.html command to
re-define what temperature or pressure compute is used for default
thermodynamic output.
:line
4.16 Walls :link(4_17),h4
Walls in an MD simulation are typically used to bound particle motion,
i.e. to serve as a boundary condition.
Walls in LAMMPS can be of rough (made of particles) or idealized
surfaces. Ideal walls can be smooth, generating forces only in the
normal direction, or frictional, generating forces also in the
tangential direction.
Rough walls, built of particles, can be created in various ways. The
particles themselves can be generated like any other particle, via the
"lattice"_lattice.html and "create_atoms"_create_atoms.html commands,
or read in via the "read_data"_read_data.html command.
Their motion can be constrained by many different commands, so that
they do not move at all, move together as a group at constant velocity
or in response to a net force acting on them, move in a prescribed
fashion (e.g. rotate around a point), etc. Note that if a time
integration fix like "fix nve"_fix_nve.html or "fix nvt"_fix_nvt.html
is not used with the group that contains wall particles, their
positions and velocities will not be updated.
"fix aveforce"_fix_aveforce.html - set force on particles to average value, so they move together
"fix setforce"_fix_setforce.html - set force on particles to a value, e.g. 0.0
"fix freeze"_fix_freeze.html - freeze particles for use as granular walls
"fix nve/noforce"_fix_nve_noforce.html - advect particles by their velocity, but without force
"fix move"_fix_move.html - prescribe motion of particles by a linear velocity, oscillation, rotation, variable :ul
The "fix move"_fix_move.html command offers the most generality, since
the motion of individual particles can be specified with
"variable"_variable.html formula which depends on time and/or the
particle position.
For rough walls, it may be useful to turn off pairwise interactions
between wall particles via the "neigh_modify
exclude"_neigh_modify.html command.
Rough walls can also be created by specifying frozen particles that do
not move and do not interact with mobile particles, and then tethering
other particles to the fixed particles, via a "bond"_bond_style.html.
The bonded particles do interact with other mobile particles.
Idealized walls can be specified via several fix commands. "Fix
wall/gran"_fix_wall_gran.html creates frictional walls for use with
granular particles; all the other commands create smooth walls.
"fix wall/reflect"_fix_wall_reflect.html - reflective flat walls
"fix wall/lj93"_fix_wall.html - flat walls, with Lennard-Jones 9/3 potential
"fix wall/lj126"_fix_wall.html - flat walls, with Lennard-Jones 12/6 potential
"fix wall/colloid"_fix_wall.html - flat walls, with "pair_style colloid"_pair_colloid.html potential
"fix wall/harmonic"_fix_wall.html - flat walls, with repulsive harmonic spring potential
"fix wall/region"_fix_wall_region.html - use region surface as wall
"fix wall/gran"_fix_wall_gran.html - flat or curved walls with "pair_style granular"_pair_gran.html potential :ul
The {lj93}, {lj126}, {colloid}, and {harmonic} styles all allow the
flat walls to move with a constant velocity, or oscillate in time.
The "fix wall/region"_fix_wall_region.html command offers the most
generality, since the region surface is treated as a wall, and the
geometry of the region can be a simple primitive volume (e.g. a
sphere, or cube, or plane), or a complex volume made from the union
and intersection of primitive volumes. "Regions"_region.html can also
be "dynamic" meaning they move with constant velocity, oscillate, or
rotate.
The only frictional idealized walls currently in LAMMPS are flat or
curved surfaces specified by the "fix wall/gran"_fix_wall_gran.html
command. At some point we plan to allow regoin surfaces to be used as
frictional walls, as well as triangulated surfaces.
:line
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