# fix wall/srd command

## Syntax

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

-   ID, group-ID are documented in [fix](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 <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 <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

``` LAMMPS
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](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](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](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](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
::: title
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
::: title
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](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
::: title
Note
:::

The walls of this fix only interact with SRD particles, as defined by
the [fix srd](fix_srd) command. If you are simulating a mixture
containing other kinds of particles, then you should typically use
[another wall command](fix_wall) 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](units) command, e.g. Angstroms for units = real or metal. A
*lattice* value means the distance units are in lattice spacings. The
[lattice](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).

``` LAMMPS
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))

------------------------------------------------------------------------

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

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

This fix computes a global array of values which can be accessed by
various [output commands](Howto_output). 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](run) command. This fix is not invoked during [energy
minimization](minimize).

## Restrictions

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

## Related commands

[fix srd](fix_srd)

## Default

none