\(\renewcommand{\AA}{\text{Å}}\)

# fix wall/srd command

## Syntax

    fix ID group-ID wall/srd face arg ... keyword value ...

- ID, group-ID are documented in fix command

- wall/srd = style name of this fix command

- one or more face/arg pairs may be appended

- face = xlo or xhi or ylo or yhi or zlo or zhi

      xlo,ylo,zlo arg = EDGE or constant or variable
        EDGE = current lo edge of simulation box
        constant = number like 0.0 or -30.0 (distance units)
        variable = equal-style variable like v_x or v_wiggle
      xhi,yhi,zhi arg = EDGE or constant or variable
        EDGE = current hi edge of simulation box
        constant = number like 50.0 or 100.3 (distance units)
        variable = equal-style variable like v_x or v_wiggle

- zero or more keyword/value pairs may be appended

- keyword = units

      units value = lattice or box
        lattice = the wall position is defined in lattice units
        box = the wall position is defined in simulation box units

## Examples

    fix xwalls all wall/srd xlo EDGE xhi EDGE
    fix walls all wall/srd xlo 0.0 ylo 10.0 units box
    fix top all wall/srd zhi v_pressdown

## Description

Bound the simulation with one or more walls which interact with stochastic reaction dynamics (SRD) particles as slip (smooth) or no-slip (rough) flat surfaces. The wall interaction is actually invoked via the fix srd command, only on the group of SRD particles it defines, so the group setting for the fix wall/srd command is ignored.

A particle/wall collision occurs if an SRD particle moves outside the wall on a timestep. This alters the position and velocity of the SRD particle and imparts a force to the wall.

The collision and Tsrd settings specified via the fix srd command affect the SRD/wall collisions. A slip setting for the collision keyword means that the tangential component of the SRD particle momentum is preserved. Thus only a normal force is imparted to the wall. The normal component of the new SRD velocity is sampled from a Gaussian distribution at temperature Tsrd.

For a noslip setting of the collision keyword, both the normal and tangential components of the new SRD velocity are sampled from a Gaussian distribution at temperature Tsrd. Additionally, a new tangential direction for the SRD velocity is chosen randomly. This collision style imparts both a normal and tangential force to the wall.

Up to 6 walls or faces can be specified in a single command: xlo, xhi, ylo, yhi, zlo, zhi. A lo face reflects particles that move to a coordinate less than the wall position, back in the hi direction. A hi face reflects particles that move to a coordinate higher than the wall position, back in the lo direction.

The position of each wall can be specified in one of 3 ways: as the EDGE of the simulation box, as a constant value, or as a variable. If EDGE is used, then the corresponding boundary of the current simulation box is used. If a numeric constant is specified then the wall is placed at that position in the appropriate dimension (x, y, or z). In both the EDGE and constant cases, the wall will never move. If the wall position is a variable, it should be specified as v_name, where name is an equal-style variable name. In this case the variable is evaluated each timestep and the result becomes the current position of the reflecting wall. Equal-style variables can specify formulas with various mathematical functions, and include thermo_style command keywords for the simulation box parameters and timestep and elapsed time. Thus it is easy to specify a time-dependent wall position.

Note

Because the trajectory of the SRD particle is tracked as it collides with the wall, you must ensure that r = distance of the particle from the wall, is always > 0 for SRD particles, or LAMMPS will generate an error. This means you cannot start your simulation with SRD particles at the wall position coord (r = 0) or with particles on the wrong side of the wall (r < 0).

Note

If you have 2 or more walls that come together at an edge or corner (e.g. walls in the x and y dimensions), then be sure to set the overlap keyword to yes in the fix srd command, since the walls effectively overlap when SRD particles collide with them. LAMMPS will issue a warning if you do not do this.

Note

The walls of this fix only interact with SRD particles, as defined by the fix srd command. If you are simulating a mixture containing other kinds of particles, then you should typically use another wall command to act on the other particles. Since SRD particles will be colliding both with the walls and the other particles, it is important to ensure that the other particle’s finite extent does not overlap an SRD wall. If you do not do this, you may generate errors when SRD particles end up “inside” another particle or a wall at the beginning of a collision step.

The units keyword determines the meaning of the distance units used to define a wall position, but only when a numeric constant is used. It is not relevant when EDGE or a variable is used to specify a face position.

A box value selects standard distance units as defined by the units command, e.g. Angstroms for units = real or metal. A lattice value means the distance units are in lattice spacings. The lattice command must have been previously used to define the lattice spacings.

Here are examples of variable definitions that move the wall position in a time-dependent fashion using equal-style variables.

    variable ramp equal ramp(0,10)
    fix 1 all wall/srd xlo v_ramp

    variable linear equal vdisplace(0,20)
    fix 1 all wall/srd xlo v_linear

    variable wiggle equal swiggle(0.0,5.0,3.0)
    fix 1 all wall/srd xlo v_wiggle

    variable wiggle equal cwiggle(0.0,5.0,3.0)
    fix 1 all wall/srd xlo v_wiggle

The ramp(lo,hi) function adjusts the wall position linearly from lo to hi over the course of a run. The vdisplace(c0,velocity) function does something similar using the equation position = c0 + velocity*delta, where delta is the elapsed time.

The swiggle(c0,A,period) function causes the wall position to oscillate sinusoidally according to this equation, where omega = 2 PI / period:

    position = c0 + A sin(omega*delta)

The cwiggle(c0,A,period) function causes the wall position to oscillate sinusoidally according to this equation, which will have an initial wall velocity of 0.0, and thus may impose a gentler perturbation on the particles:

    position = c0 + A (1 - cos(omega*delta))

## Dump image info

Added in version 11Feb2026.

Fix wall/srd supports the fix keyword of dump image. The fix will pass geometry information about the 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 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 adjust the radius of the rendered cylinder. It should be set to a value > 0 or the cylinder will not be visible since the diameter is set internally to zero due to lack of a suitable heuristic for deriving a meaningful diameter for all types of walls and unit settings.

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

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

No information about this fix is written to binary restart files. None of the fix_modify options are relevant to this fix.

This fix computes a global array of values which can be accessed by various output commands. The number of rows in the array is equal to the number of walls defined by the fix. The number of columns is 3, for the x,y,z components of force on each wall.

Note that an outward normal force on a wall will be a negative value for lo walls and a positive value for hi walls. The array values calculated by this fix are “extensive”.

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

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

## Related commands

fix srd

## Default

none