1.. index:: fix wall/ees
2.. index:: fix wall/region/ees
3
4fix wall/ees command
5====================
6
7fix wall/region/ees command
8===========================
9
10Syntax
11""""""
12
13.. parsed-literal::
14
15   fix ID group-ID style args
16
17* ID, group-ID are documented in :doc:`fix <fix>` command
18* style = *wall/ees* or *wall/region/ees*
19
20  .. parsed-literal::
21
22       args for style *wall/ees*\ : one or more *face parameters* groups may be appended
23       face = *xlo* or *xhi* or *ylo* or *yhi* or *zlo* or *zhi*
24       parameters = coord epsilon sigma cutoff
25         coord = position of wall = EDGE or constant or variable
26           EDGE = current lo or hi edge of simulation box
27           constant = number like 0.0 or -30.0 (distance units)
28           variable = :doc:`equal-style variable <variable>` like v_x or v_wiggle
29         epsilon = strength factor for wall-particle interaction (energy or energy/distance\^2 units)
30           epsilon can be a variable (see below)
31         sigma = size factor for wall-particle interaction (distance units)
32           sigma can be a variable (see below)
33         cutoff = distance from wall at which wall-particle interaction is cut off (distance units)
34
35  .. parsed-literal::
36
37       args for style *wall/region/ees*\ : *region-ID* *epsilon* *sigma* *cutoff*
38         region-ID = region whose boundary will act as wall
39         epsilon = strength factor for wall-particle interaction (energy or energy/distance\^2 units)
40         sigma = size factor for wall-particle interaction (distance units)
41         cutoff = distance from wall at which wall-particle interaction is cut off (distance units)
42
43Examples
44""""""""
45
46.. code-block:: LAMMPS
47
48   fix wallhi all wall/ees xlo -1.0 1.0 1.0 2.5 units box
49   fix wallhi all wall/ees xhi EDGE 1.0 1.0 2.5
50   fix wallhi all wall/ees v_wiggle 23.2 1.0 1.0 2.5
51   fix zwalls all wall/ees zlo 0.0 1.0 1.0 0.858 zhi 40.0 1.0 1.0 0.858
52
53   fix ees_cube all wall/region/ees myCube 1.0 1.0 2.5
54
55Description
56"""""""""""
57
58Fix *wall/ees* bounds the simulation domain on one or more of its
59faces with a flat wall that interacts with the ellipsoidal atoms in
60the group by generating a force on the atom in a direction
61perpendicular to the wall and a torque parallel with the wall.  The
62energy of wall-particle interactions E is given by:
63
64.. math::
65
66   E = \epsilon \left[ \frac{2 \sigma_{LJ}^{12} \left(7 r^5+14 r^3
67   \sigma_{n}^2+3 r \sigma_{n}^4\right) }{945
68   \left(r^2-\sigma_{n}^2\right)^7} -\frac{ \sigma_{LJ}^6 \left(2 r
69   \sigma_{n}^3+\sigma_{n}^2 \left(r^2-\sigma_{n}^2\right)\log{
70   \left[\frac{r-\sigma_{n}}{r+\sigma_{n}}\right]}\right) }{12
71   \sigma_{n}^5 \left(r^2-\sigma_{n}^2\right)} \right]\qquad \sigma_n
72   < r < r_c
73
74Introduced by Babadi and Ejtehadi in :ref:`(Babadi)
75<BabadiEjtehadi>`. Here, *r* is the distance from the particle to the
76wall at position *coord*, and Rc is the *cutoff* distance at which
77the particle and wall no longer interact. Also, :math:`\sigma_n` is
78the distance between center of ellipsoid and the nearest point of its
79surface to the wall as shown below.
80
81.. image:: JPG/fix_wall_ees_image.jpg
82   :align: center
83
84Details of using this command and specifications are the same as
85fix/wall command. You can also find an example in USER/ees/ under
86examples/ directory.
87
88The prefactor :math:`\epsilon` can be thought of as an
89effective Hamaker constant with energy units for the strength of the
90ellipsoid-wall interaction.  More specifically, the :math:`\epsilon`
91pre-factor is
92
93.. math::
94
95   8 \pi^2 \quad \rho_{wall} \quad \rho_{ellipsoid} \quad \epsilon
96   \quad \sigma_a \quad \sigma_b \quad \sigma_c
97
98where :math:`\epsilon` is the LJ energy parameter for the constituent
99LJ particles and :math:`\sigma_a`, :math:`\sigma_b`, and
100:math:`\sigma_c` are the radii of the ellipsoidal
101particles. :math:`\rho_{wall}` and :math:`\rho_{ellipsoid}` are the
102number density of the constituent particles, in the wall and ellipsoid
103respectively, in units of 1/volume.
104
105.. note::
106
107   You must insure that r is always bigger than :math:`\sigma_n` for
108   all particles in the group, or LAMMPS will generate an error.  This
109   means you cannot start your simulation with particles touching the wall
110   position *coord* (:math:`r = \sigma_n`) or with particles penetrating
111   the wall (:math:`0 =< r < \sigma_n`) or with particles on the wrong
112   side of the wall (:math:`r < 0`).
113
114Fix *wall/region/ees* treats the surface of the geometric region defined
115by the *region-ID* as a bounding wall which interacts with nearby
116ellipsoidal particles according to the EES potential introduced above.
117
118Other details of this command are the same as for the :doc:`fix
119wall/region <fix_wall_region>` command.  One may also find an example
120of using this fix in the examples/PACKAGES/ees/ directory.
121
122----------
123
124Restart, fix_modify, output, run start/stop, minimize info
125"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
126
127No information about these fixes are written to :doc:`binary restart
128files <restart>`.
129
130The :doc:`fix_modify <fix_modify>` *energy* option is supported by
131these fixes to add the energy of interaction between atoms and all the
132specified walls or region wall to the global potential energy of the
133system as part of :doc:`thermodynamic output <thermo_style>`.  The
134default settings for these fixes are :doc:`fix_modify energy no
135<fix_modify>`.
136
137The :doc:`fix_modify <fix_modify>` *respa* option is supported by
138these fixes. This allows to set at which level of the :doc:`r-RESPA
139<run_style>` integrator the fix is adding its forces. Default is the
140outermost level.
141
142These fixes computes a global scalar and a global vector of forces,
143which can be accessed by various :doc:`output commands
144<Howto_output>`.  See the :doc:`fix wall <fix_wall>` command for a
145description of the scalar and vector.
146
147No parameter of these fixes can be used with the *start/stop* keywords of
148the :doc:`run <run>` command.
149
150The forces due to these fixes are imposed during an energy
151minimization, invoked by the :doc:`minimize <minimize>` command.
152
153.. note::
154
155   If you want the atom/wall interaction energy to be included in
156   the total potential energy of the system (the quantity being
157   minimized), you MUST enable the :doc:`fix_modify <fix_modify>` *energy*
158   option for this fix.
159
160Restrictions
161""""""""""""
162
163These fixes are part of the EXTRA-FIX package.  They are only enabled
164if LAMMPS was built with that package.  See the :doc:`Build package
165<Build_package>` page for more info.
166
167These fixes requires that atoms be ellipsoids as defined by the
168:doc:`atom_style ellipsoid <atom_style>` command.
169
170Related commands
171""""""""""""""""
172
173:doc:`fix wall <fix_wall>`,
174:doc:`pair resquared <pair_resquared>`
175
176Default
177"""""""
178
179none
180
181----------
182
183.. _BabadiEjtehadi:
184
185**(Babadi)** Babadi and Ejtehadi, EPL, 77 (2007) 23002.
186