.. index:: pair_style gauss
.. index:: pair_style gauss/gpu
.. index:: pair_style gauss/omp
.. index:: pair_style gauss/cut
.. index:: pair_style gauss/cut/omp

pair_style gauss command
========================

Accelerator Variants: *gauss/gpu*, *gauss/omp*

pair_style gauss/cut command
============================

Accelerator Variants: *gauss/cut/omp*

Syntax
""""""

.. code-block:: LAMMPS

   pair_style gauss cutoff
   pair_style gauss/cut cutoff

* cutoff = global cutoff for Gauss interactions (distance units)

Examples
""""""""

.. code-block:: LAMMPS

   pair_style gauss 12.0
   pair_coeff * * 1.0 0.9
   pair_coeff 1 4 1.0 0.9 10.0

   pair_style gauss/cut 3.5
   pair_coeff 1 4 0.2805 1.45 0.112

Description
"""""""""""

Style *gauss* computes a tethering potential of the form

.. math::

   E = - A \exp(-B r^2) \qquad r < r_c

between an atom and its corresponding tether site which will typically
be a frozen atom in the simulation.  :math:`r_c` is the cutoff.

The following coefficients must be defined for each pair of atom types
via the :doc:`pair_coeff <pair_coeff>` command as in the examples above,
or in the data file or restart files read by the
:doc:`read_data <read_data>` or :doc:`read_restart <read_restart>`
commands:

* A (energy units)
* B (1/distance\^2 units)
* cutoff (distance units)

The last coefficient is optional. If not specified, the global cutoff
is used.

Style *gauss/cut* computes a generalized Gaussian interaction potential
between pairs of particles:

.. math::

   E = \frac{H}{\sigma_h\sqrt{2\pi}} \exp\left[-\frac{(r-r_{mh})^2}{2\sigma_h^2}\right]

where H determines together with the standard deviation :math:`\sigma_h`
the peak height of the Gaussian function, and :math:`r_{mh}` the peak
position.  Examples of the use of the Gaussian potentials include
implicit solvent simulations of salt ions :ref:`(Lenart) <Lenart2>` and
of surfactants :ref:`(Jusufi) <Jusufi2>`.  In these instances the
Gaussian potential mimics the hydration barrier between a pair of
particles. The hydration barrier is located at :math:`r_{mh}` and has a
width of :math:`\sigma_h`. The prefactor determines the height of the
potential barrier.

The following coefficients must be defined for each pair of atom types
via the :doc:`pair_coeff <pair_coeff>` command as in the example above,
or in the data file or restart files read by the
:doc:`read_data <read_data>` or :doc:`read_restart <read_restart>`
commands:

* H (energy \* distance units)
* :math:`r_{mh}` (distance units)
* :math:`\sigma_h` (distance units)
* cutoff (distance units)

The last coefficient is optional. If not specified, the global cutoff
is used.

----------

.. include:: accel_styles.rst

----------

Mixing, shift, table, tail correction, restart, rRESPA info
"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""

For atom type pairs I,J and I != J, the A, B, H, sigma_h, r_mh
parameters, and the cutoff distance for these pair styles can be mixed:
A (energy units)
sqrt(1/B) (distance units, see below)
H (energy units)
sigma_h (distance units)
r_mh (distance units)
cutoff (distance units):ul

The default mix value is *geometric*\ .
Only *arithmetic* and *geometric* mix values are supported.
See the "pair_modify" command for details.

The A and H parameters are mixed using the same rules normally
used to mix the "epsilon" parameter in a Lennard Jones interaction.
The sigma_h, r_mh, and the cutoff distance are mixed using the same
rules used to mix the "sigma" parameter in a Lennard Jones interaction.
The B parameter is converted to a distance (sigma), before mixing
(using sigma=B\^-0.5), and converted back to a coefficient
afterwards (using B=sigma\^2).
Negative A values are converted to positive A values (using abs(A))
before mixing, and converted back after mixing
(by multiplying by min(sign(Ai),sign(Aj))).
This way, if either particle is repulsive (if Ai<0 or Aj<0),
then the default interaction between both particles will be repulsive.

The *gauss* style does not support the :doc:`pair_modify <pair_modify>`
shift option. There is no effect due to the Gaussian well beyond the
cutoff; hence reasonable cutoffs need to be specified.

The *gauss/cut* style supports the :doc:`pair_modify <pair_modify>` shift
option for the energy of the Gauss-potential portion of the pair
interaction.

The :doc:`pair_modify <pair_modify>` table and tail options are not
relevant for these pair styles.

These pair styles write their information to :doc:`binary restart files <restart>`, so pair_style and pair_coeff commands do not need
to be specified in an input script that reads a restart file.

These pair styles can only be used via the *pair* keyword of the
:doc:`run_style respa <run_style>` command.  They do not support the
*inner*, *middle*, *outer* keywords.

The *gauss* pair style tallies an "occupancy" count of how many Gaussian-well
sites have an atom within the distance at which the force is a maximum
= sqrt(0.5/b).  This quantity can be accessed via the :doc:`compute pair <compute_pair>` command as a vector of values of length 1.

To print this quantity to the log file (with a descriptive column
heading) the following commands could be included in an input script:

.. code-block:: LAMMPS

   compute gauss all pair gauss
   variable occ equal c_gauss[1]
   thermo_style custom step temp epair v_occ

----------

Restrictions
""""""""""""

The *gauss* and *gauss/cut* styles are part of the EXTRA-PAIR package.
They are only enabled if LAMMPS is build with that package.  See the
:doc:`Build package <Build_package>` page for more info.

The *gauss* style does not apply :doc:`special_bonds <special_bonds>`
factors. When using this pair style on a system that has bonds, the
special_bonds factors, if using the default setting of 0.0, may need to
be adjusted to some very small number (e.g. 1.0e-100), so that those
special pairs are not completely excluded from the neighbor lists, but
won't contribute forces or energies from styles (e.g. when used in
combination with a :doc:`hybrid pair style <pair_hybrid>`) that do
apply those factors.

Related commands
""""""""""""""""

:doc:`pair_coeff <pair_coeff>`,
:doc:`pair_style coul/diel <pair_coul_diel>`

Default
"""""""

none

.. _Lenart2:

**(Lenart)** Lenart , Jusufi, and Panagiotopoulos, J Chem Phys, 126,
044509 (2007).

.. _Jusufi2:

**(Jusufi)** Jusufi, Hynninen, and Panagiotopoulos, J Phys Chem B, 112,
13783 (2008).
