# fix wall/region command

## Syntax

    fix ID group-ID wall/region region-ID style args ... cutoff

-   ID, group-ID are documented in [fix](fix) command

-   wall/region = style name of this fix command

-   region-ID = region whose boundary will act as wall

-   style = *lj93* or *lj126* or *lj1043* or *colloid* or *harmonic* or
    *morse*

-   args for styles *lj93* or *lj126* or *lj1043* or *colloid* or
    *harmonic* =

        epsilon = strength factor for wall-particle interaction (energy or energy/distance\^2 units)
        sigma = size factor for wall-particle interaction (distance units)

-   args for style *morse* =

        D_0 = depth of the potential (energy units)
        alpha = width parameter (1/distance units)
        r_0 = distance of the potential minimum from wall position (distance units)

-   cutoff = distance from wall at which wall-particle interaction is
    cut off (distance units)

## Examples

``` LAMMPS
fix wall all wall/region mySphere lj93 1.0 1.0 2.5
fix wall all wall/region mySphere harmonic 1.0 0.0 2.5
fix wall all wall/region box_top morse 1.0 1.0 1.5 3.0
```

## Description

Treat the surface of the geometric region defined by the *region-ID* as
a bounding wall which interacts with nearby particles according to the
specified style.

The distance between a particle and the surface is the distance to the
nearest point on the surface and the force the wall exerts on the
particle is along the direction between that point and the particle,
which is the direction normal to the surface at that point. Note that if
the region surface is comprised of multiple \"faces\", then each face
can exert a force on the particle if it is close enough. E.g. for
[region_style block](region), a particle in the interior, near a corner
of the block, could feel wall forces from 1, 2, or 3 faces of the block.

Regions are defined using the [region](region) command. Note that the
region volume can be interior or exterior to the bounding surface, which
will determine in which direction the surface interacts with particles,
i.e. the direction of the surface normal. The surface of the region only
exerts forces on particles \"inside\" the region; if a particle is
\"outside\" the region it will generate an error, because it has moved
through the wall.

Regions can either be primitive shapes (block, sphere, cylinder, etc) or
combinations of primitive shapes specified via the *union* or
*intersect* region styles. These latter styles can be used to construct
particle containers with complex shapes. Regions can also change over
time via the [region](region) command keywords (move) and *rotate*. If
such a region is used with this fix, then the of region surface will
move over time in the corresponding manner.

:::: note
::: title
Note
:::

As discussed on the [region](region) command doc page, regions in LAMMPS
do not get wrapped across periodic boundaries. It is up to you to ensure
that periodic or non-periodic boundaries are specified appropriately via
the [boundary](boundary) command when using a region as a wall that
bounds particle motion. This also means that if you embed a region in
your simulation box and want it to repulse particles from its surface
(using the \"side out\" option in the [region](region) command), that
its repulsive force will not be felt across a periodic boundary.
::::

:::: note
::: title
Note
:::

For primitive regions with sharp corners and/or edges (e.g. a block or
cylinder), wall/particle forces are computed accurately for both
interior and exterior regions. For *union* and *intersect* regions,
additional sharp corners and edges may be present due to the
intersection of the surfaces of 2 or more primitive volumes. These
corners and edges can be of two types: concave or convex. Concave
points/edges are like the corners of a cube as seen by particles in the
interior of a cube. Wall/particle forces around these features are
computed correctly. Convex points/edges are like the corners of a cube
as seen by particles exterior to the cube, i.e. the points jut into the
volume where particles are present. LAMMPS does NOT compute the location
of these convex points directly, and hence wall/particle forces in the
cutoff volume around these points suffer from inaccuracies. The basic
problem is that the outward normal of the surface is not continuous at
these points. This can cause particles to feel no force (they don\'t
\"see\" the wall) when in one location, then move a distance epsilon,
and suddenly feel a large force because they now \"see\" the wall. In a
worst-case scenario, this can blow particles out of the simulation box.
Thus, as a general rule you should not use the fix wall/gran/region
command with *union* or *interesect* regions that have convex points or
edges resulting from the union/intersection (convex points/edges in the
union/intersection due to a single sub-region are still OK).
::::

:::: note
::: title
Note
:::

Similarly, you should not define *union* or *intersert* regions for use
with this command that share an overlapping common face that is part of
the overall outer boundary (interior boundary is OK), even if the face
is smooth. E.g. two regions of style block in a *union* region, where
the two blocks overlap on one or more of their faces. This is because
LAMMPS discards points that are part of multiple sub-regions when
calculating wall/particle interactions, to avoid double-counting the
interaction. Having two coincident faces could cause the face to become
invisible to the particles. The solution is to make the two faces differ
by epsilon in their position.
::::

The energy of wall-particle interactions depends on the specified style.

For style *lj93*, the energy E is given by the 9/3 potential:

$$E = \epsilon \left[ \frac{2}{15} \left(\frac{\sigma}{r}\right)^{9} -
                      \left(\frac{\sigma}{r}\right)^3 \right]
                      \qquad r < r_c$$

For style *lj126*, the energy E is given by the 12/6 potential:

$$E = 4 \epsilon \left[ \left(\frac{\sigma}{r}\right)^{12} -
                      \left(\frac{\sigma}{r}\right)^6 \right]
                      \qquad r < r_c$$

For style *wall/lj1043*, the energy E is given by the 10/4/3 potential:

$$E = 2 \pi \epsilon \left[ \frac{2}{5} \left(\frac{\sigma}{r}\right)^{10} -
                      \left(\frac{\sigma}{r}\right)^4 -
                      \frac{\sqrt(2)\sigma^3}{3\left(r+\left(0.61/\sqrt(2)\right)\sigma\right)^3}\right]
                      \qquad r < r_c$$

For style *colloid*, the energy E is given by an integrated form of the
[pair_style colloid](pair_colloid) potential:

$$\begin{aligned}
E = & \epsilon \left[ \frac{\sigma^{6}}{7560}
\left(\frac{6R-D}{D^{7}} + \frac{D+8R}{(D+2R)^{7}} \right) \right. \\
 & \left. - \frac{1}{6} \left(\frac{2R(D+R) + D(D+2R)
 \left[ \ln D - \ln (D+2R) \right]}{D(D+2R)} \right) \right] \qquad r < r_c
\end{aligned}$$

For style *wall/harmonic*, the energy E is given by a harmonic spring
potential (the distance parameter is ignored):

$$E = \epsilon \quad (r - r_c)^2 \qquad r < r_c$$

For style *wall/morse*, the energy E is given by the Morse potential:

$$E = D_0 \left[ e^{- 2 \alpha (r - r_0)} - 2 e^{- \alpha (r - r_0)} \right]
    \qquad r < r_c$$

Unlike other styles, this requires three parameters ($D_0$, $\alpha$,
and $r_0$ in this order) instead of two like for the other wall styles.

In all cases, *r* is the distance from the particle to the region
surface, and Rc is the *cutoff* distance at which the particle and
surface no longer interact. The cutoff is always the last argument. The
energy of the wall potential is shifted so that the wall-particle
interaction energy is 0.0 at the cutoff distance.

For a full description of these wall styles, see fix_style
[wall](fix_wall)

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

No information about this fix is written to [binary restart
files](restart).

The [fix_modify](fix_modify) *energy* option is supported by this fix to
add the energy of interaction between atoms and the region wall to the
global potential energy of the system as part of [thermodynamic
output](thermo_style). The default setting for this fix is [fix_modify
energy no](fix_modify).

The [fix_modify](fix_modify) *virial* option is supported by this fix to
add the contribution due to the interaction between atoms and the region
wall to both the global pressure and per-atom stress of the system via
the [compute pressure](compute_pressure) and [compute
stress/atom](compute_stress_atom) commands. The former can be accessed
by [thermodynamic output](thermo_style). The default setting for this
fix is [fix_modify virial no](fix_modify).

The [fix_modify](fix_modify) *respa* option is supported by this fix.
This allows to set at which level of the [r-RESPA](run_style) integrator
the fix is adding its forces. Default is the outermost level.

This fix computes a global scalar energy and a global 3-length vector of
forces, which can be accessed by various [output
commands](Howto_output). The scalar energy is the sum of energy
interactions for all particles interacting with the wall represented by
the region surface. The 3 vector quantities are the x,y,z components of
the total force acting on the wall due to the particles. The scalar and
vector 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.

The forces due to this fix are imposed during an energy minimization,
invoked by the [minimize](minimize) command.

:::: note
::: title
Note
:::

If you want the atom/wall interaction energy to be included in the total
potential energy of the system (the quantity being minimized), you MUST
enable the [fix_modify](fix_modify) *energy* option for this fix.
::::

## Restrictions

> none

## Related commands

[fix wall/lj93](fix_wall), [fix wall/lj126](fix_wall), [fix
wall/lj1043](fix_wall), [fix wall/colloid](fix_wall), [fix
wall/harmonic](fix_wall), [fix wall/gran](fix_wall_gran)

## Default

none