\(\renewcommand{\AA}{\text{Å}}\)

fix wall/body/polyhedron command

Syntax

fix ID group-ID wall/body/polyhedron k_n c_n c_t wallstyle args keyword values ...

• ID, group-ID are documented in fix command

• wall/body/polyhedron = style name of this fix command

• k_n = normal repulsion strength (force/distance units or pressure units - see discussion below)

• c_n = normal damping coefficient (force/distance units or pressure units - see discussion below)

• c_t = tangential damping coefficient (force/distance units or pressure units - see discussion below)

• wallstyle = xplane or yplane or zplane

• args = list of arguments for a particular style

  xplane or yplane or zplane args = lo hi
    lo,hi = position of lower and upper plane (distance units), either can be NULL)

• zero or more keyword/value pairs may be appended to args

• keyword = wiggle or history

  wiggle values = dim amplitude period
    dim = x or y or z
    amplitude = size of oscillation (distance units)
    period = time of oscillation (time units)
  history values = mu k_t
    mu = friction coefficient between the wall and the particles
    k_t = tangential stiffness (same units as k_n), or NULL for 2/7 k_n

Examples

fix 1 all wall/body/polyhedron 1000.0 20.0 5.0 xplane -10.0 10.0
fix 1 all wall/body/polyhedron 1000.0 20.0 5.0 yplane 0.0 NULL history 0.5 NULL

Description

This fix is for use with 3d models of body particles of style rounded/polyhedron. It bounds the simulation domain with wall(s). All particles in the group interact with the wall when they are close enough to touch it. The nature of the interaction between the wall and the polygon particles is the same as that between the polygon particles themselves, which is similar to a Hookean potential. See the Howto body page for more details on using body particles.

The parameters k_n, c_n, c_t have the same meaning and units as those specified with the pair_style body/rounded/polyhedron command.

Changed in version 30Sep2026.

Each vertex of a particle, or the center of a sphere, is repelled by the wall with the force \(k_n (r_v - d)\), when its signed distance \(d\) from the wall along the wall normal is smaller than its rounded radius \(r_v\), also when the vertex has moved past the wall. The damping forces act at the contact point between the rounded surface and the wall, using the velocity of the particle at that point relative to the wall, so that they also exert torques. There is no cohesion with the wall, and no friction force unless the history keyword is used (see below), and the contact forces are not scaled by the size of the contact region. Previously, a vertex that had moved past the wall was no longer repelled, or even pushed further out. A particle interacts with both walls of a pair, e.g. in a narrow channel. Previously, only the wall nearer to the center of the particle was checked, so that an elongated particle could overlap the other wall without being repelled.

The wallstyle is planar. The 3 options specify a pair of walls in a dimension. Wall positions are given by lo and hi. Either of the values can be specified as NULL if a single wall is desired.

Optionally, the wall can be moving, if the wiggle keyword is appended.

For the wiggle keyword, the wall oscillates sinusoidally, similar to the oscillations of particles which can be specified by the fix move command. This is useful in packing simulations of particles. The arguments to the wiggle keyword specify a dimension for the motion, as well as its amplitude and period. Note that if the dimension is in the plane of the wall, this is effectively a shearing motion. If the dimension is perpendicular to the wall, it is more of a shaking motion.

Each timestep, the position of a wiggled wall in the appropriate dim is set according to this equation:

position = coord + A - A cos (omega * delta)

where coord is the specified initial position of the wall, A is the amplitude, omega is 2 PI / period, and delta is the time elapsed since the fix was specified. The velocity of the wall is set to the derivative of this expression.

Added in version 30Sep2026.

With the history keyword, the wall also exerts a friction force from a tangential spring on the particles, similar to the history keyword of pair_style body/rounded/polyhedron between two particles. The tangential deformation \(\xi\) of a particle touching the wall is accumulated from its tangential velocity relative to the wall, \(\xi \leftarrow \xi + v_t \Delta t\), and the friction force is \(-k_t \xi\), with a magnitude of at most \(\mu F_n\), where \(F_n\) is the sum of the elastic normal forces of all vertices of the particle touching the wall. Once the spring force exceeds this limit, the particle slides and \(\xi\) is reduced accordingly. The friction force acts at the average of the contact points of these vertices, weighted by their elastic normal forces, and \(v_t\) is the velocity of the particle at that point, so that a particle resting with a face on the wall is not subject to a spurious torque. The tangential deformation is reset to zero when the particle no longer touches the wall. A particle touching both walls of a pair has a separate tangential deformation and friction force for each wall. If k_t is specified as NULL, it is set to \(\frac{2}{7} k_n\) as for the pair style. The damping forces are unchanged. This allows particles to stay at rest on the wall, e.g. under gravity that is inclined with respect to the wall.

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Dump image info

Added in version 11Feb2026.

This fix supports the fix keyword of dump image. The fix will pass geometry information about xplane, yplane, and zplane style walls to dump image so that the walls will be included in the rendered image. Please note, that for 2d systems, a wall rendered as a plane would be invisible and it is thus rendered as a cylinder.

The color of the wall is by default that of the first atom type when using color styles “type” or “element”. With color style “const” the default value of “white” can be changed using dump_modify fcolor. The transparency is by default fully opaque and can be changed globally with dump_modify ftrans.

For 2d systems, the fflag1 setting determines whether the cylinder representing the wall is capped with a sphere at the ends: 0 means no caps, 1 means the lower end is capped, 2 means the upper end is capped, and 3 means both ends are capped. The fflag2 setting allows to set the radius of the rendered cylinders.

For 3d systems, both fflag1 and fflag2 are ignored.

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Restart, fix_modify, output, run start/stop, minimize info

With the history keyword, this fix writes the tangential deformations of the particles touching the wall to binary restart files, so that a simulation can continue correctly. See the read_restart command for info on how to re-specify a fix in an input script that reads a restart file, so that the operation of the fix continues in an uninterrupted fashion. Otherwise no information about this fix is written to binary restart files.

None of the fix_modify options are relevant to this fix. No global or per-atom quantities are stored by this fix for access by various output commands. No parameter of this fix can be used with the start/stop keywords of the run command. This fix is not invoked during energy minimization.

Restrictions

This fix is part of the BODY package. It is only enabled if LAMMPS was built with that package. See the Build package page for more info.

Any dimension (xyz) that has a wall must be non-periodic.

Related commands

atom_style body, pair_style body/rounded/polyhedron

Default

The history keyword is not used.