# fix wall/ees command

# fix wall/region/ees command

## Syntax

    fix ID group-ID style args

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

-   style = *wall/ees* or *wall/region/ees*

        args for style *wall/ees*\ : one or more *face parameters* groups may be appended
        face = *xlo* or *xhi* or *ylo* or *yhi* or *zlo* or *zhi*
        parameters = coord epsilon sigma cutoff
          coord = position of wall = EDGE or constant or variable
            EDGE = current lo or hi 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
          epsilon = strength factor for wall-particle interaction (energy or energy/distance\^2 units)
            epsilon can be a variable (see below)
          sigma = size factor for wall-particle interaction (distance units)
            sigma can be a variable (see below)
          cutoff = distance from wall at which wall-particle interaction is cut off (distance units)

        args for style *wall/region/ees*\ : *region-ID* *epsilon* *sigma* *cutoff*
          region-ID = region whose boundary will act as wall
          epsilon = strength factor for wall-particle interaction (energy or energy/distance\^2 units)
          sigma = size factor for wall-particle interaction (distance units)
          cutoff = distance from wall at which wall-particle interaction is cut off (distance units)

## Examples

``` LAMMPS
fix wallhi all wall/ees xlo -1.0 1.0 1.0 2.5 units box
fix wallhi all wall/ees xhi EDGE 1.0 1.0 2.5
fix wallhi all wall/ees v_wiggle 23.2 1.0 1.0 2.5
fix zwalls all wall/ees zlo 0.0 1.0 1.0 0.858 zhi 40.0 1.0 1.0 0.858

fix ees_cube all wall/region/ees myCube 1.0 1.0 2.5
```

## Description

Fix *wall/ees* bounds the simulation domain on one or more of its faces
with a flat wall that interacts with the ellipsoidal atoms in the group
by generating a force on the atom in a direction perpendicular to the
wall and a torque parallel with the wall. The energy of wall-particle
interactions E is given by:

$$E = \epsilon \left[ \frac{2 \sigma_{LJ}^{12} \left(7 r^5+14 r^3
\sigma_{n}^2+3 r \sigma_{n}^4\right) }{945
\left(r^2-\sigma_{n}^2\right)^7} -\frac{ \sigma_{LJ}^6 \left(2 r
\sigma_{n}^3+\sigma_{n}^2 \left(r^2-\sigma_{n}^2\right)\log{
\left[\frac{r-\sigma_{n}}{r+\sigma_{n}}\right]}\right) }{12
\sigma_{n}^5 \left(r^2-\sigma_{n}^2\right)} \right]\qquad \sigma_n
< r < r_c$$

Introduced by Babadi and Ejtehadi in [(Babadi)](BabadiEjtehadi). Here,
*r* is the distance from the particle to the wall at position *coord*,
and Rc is the *cutoff* distance at which the particle and wall no longer
interact. Also, $\sigma_n$ is the distance between center of ellipsoid
and the nearest point of its surface to the wall as shown below.

![image](JPG/fix_wall_ees_image.jpg){.align-center}

Details of using this command and specifications are the same as
fix/wall command. You can also find an example in USER/ees/ under
examples/ directory.

The prefactor $\epsilon$ can be thought of as an effective Hamaker
constant with energy units for the strength of the ellipsoid-wall
interaction. More specifically, the $\epsilon$ prefactor is

$$8 \pi^2 \quad \rho_{wall} \quad \rho_{ellipsoid} \quad \epsilon
\quad \sigma_a \quad \sigma_b \quad \sigma_c$$

where $\epsilon$ is the LJ energy parameter for the constituent LJ
particles and $\sigma_a$, $\sigma_b$, and $\sigma_c$ are the radii of
the ellipsoidal particles. $\rho_{wall}$ and $\rho_{ellipsoid}$ are the
number density of the constituent particles, in the wall and ellipsoid
respectively, in units of 1/volume.

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

You must ensure that r is always bigger than $\sigma_n$ for all
particles in the group, or LAMMPS will generate an error. This means you
cannot start your simulation with particles touching the wall position
*coord* ($r = \sigma_n$) or with particles penetrating the wall
($0 =< r < \sigma_n$) or with particles on the wrong side of the wall
($r < 0$).
::::

Fix *wall/region/ees* treats the surface of the geometric region defined
by the *region-ID* as a bounding wall which interacts with nearby
ellipsoidal particles according to the EES potential introduced above.

Other details of this command are the same as for the [fix
wall/region](fix_wall_region) command. One may also find an example of
using this fix in the examples/PACKAGES/ees/ directory.

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

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

No information about these fixes are written to [binary restart
files](restart).

The [fix_modify](fix_modify) *energy* option is supported by these fixes
to add the energy of interaction between atoms and all the specified
walls or region wall to the global potential energy of the system as
part of [thermodynamic output](thermo_style). The default settings for
these fixes are [fix_modify energy no](fix_modify).

The [fix_modify](fix_modify) *respa* option is supported by these fixes.
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.

These fixes computes a global scalar and a global vector of forces,
which can be accessed by various [output commands](Howto_output). See
the [fix wall](fix_wall) command for a description of the scalar and
vector.

No parameter of these fixes can be used with the *start/stop* keywords
of the [run](run) command.

The forces due to these fixes 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

These fixes are part of the EXTRA-FIX package. They are only enabled if
LAMMPS was built with that package. See the [Build
package](Build_package) page for more info.

These fixes requires that atoms be ellipsoids as defined by the
[atom_style ellipsoid](atom_style) command.

## Related commands

[fix wall](fix_wall), [pair resquared](pair_resquared)

## Default

none

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

::: {#BabadiEjtehadi}
**(Babadi)** Babadi and Ejtehadi, EPL, 77 (2007) 23002.
:::