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.. index:: pair_style ilj/cut
pair_style ilj/cut command
=============================
Syntax
""""""
.. code-block:: LAMMPS
pair_style ilj/cut cutoff
* ilj/cut = name of the pair style
* cutoff = global cutoff (distance units)
Examples
""""""""
.. code-block:: LAMMPS
pair_style ilj/cut 10.0
pair_coeff 1 1 4.0 7.0 6.0 3.5 0.0045
Description
"""""""""""
.. versionadded:: 29Aug2024
Pair style *ilj/cut* computes pairwise interactions from an Improved Lennard-Jones (ILJ) potential according to :ref:`(Pirani) <Pirani>`:
.. math::
x = r/R_m \\
n_x = \alpha*x^2 + \beta \\
\gamma \equiv m \\
V(x) = \epsilon \cdot \left( \frac{\gamma}{n_x - \gamma} \left(\frac{1}{x} \right)^{n_x} - \frac{n_x}{n_x - \gamma} \left(\frac{1}{x} \right)^{\gamma} \right)
\qquad r < r_c
:math:`r_c` is the cutoff.
An additional parameter, :math:`\alpha`, has been introduced in order to be able to recover the traditional LJ 12-6 with an adequate choice of parameters. With
:math:`R_m \equiv r_0 = \sigma \cdot 2^{1/6}`, :math:`\alpha = 0`, :math:`\beta = 12` and :math:`\gamma = 6` it is straightforward
to prove that LJ 12-6 is obtained.
This potential provides some advantages with respect to the Lennard-Jones (LJ) potential and can be really useful for molecular dynamics simulations, as one can see from :ref:`(Pirani) <Pirani>`.
It can be used for neutral-neutral (:math:`\gamma = 6`), ion-neutral (:math:`\gamma = 4`) or ion-ion systems (:math:`\gamma = 1`).
It removes most of the issues at short- and long-range of the LJ model.
It is possible to verify that using (:math:`\alpha= 4`), (:math:`\beta= 6`) and (:math:`\gamma = 6`), at the equilibrium distance,
the first and second derivatives of ILJ coincide with those of LJ 12-6 ( and the reduced force constant amounts to the typical 72).
In this case, LJ provides a long-range coefficient with a factor of 2 compared with the ILJ. Also, the short-range interaction is overestimated by LJ.
The ILJ potential solves both problems.
The analysis of a diverse amount of systems verified that (:math:`\alpha= 4`) works very well.
In some special cases (e.g. those involving very small multiple charged ions) this factor may take a slightly different value.
The parameter (:math:`\beta`) codifies the hardness (polarizability) of the interacting partners, and for neutral-neutral systems it ranges from 6 to 11.
Moreover, the modulation of (:math:`\beta`) permits to indirectly consider the role of further interaction components (such as the charge transfer int he perturbative limit)
and mitigates the effect of some uncertainty in the data.
A couple of examples of the applications of the ILJ potential should be useful to show its wide variety of uses:
- The ILJ potential has been successfully used to model the interaction between a graphdiyne (GDY) membrane and molecular hydrogen (:ref:`(Arroyo) <Arroyo>`).
- The ILJ model and the phenomenological approach have been recently adopted by the NASA Laboratory to evaluate transport properties in gaseous mixtures (:ref:`(Chatzigeorgis) <Chatzigeorgis>`)
The following coefficients must be defined for each pair of atoms
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:
* :math:`\alpha` (dimensionless)
* :math:`\beta` (dimensionless)
* :math:`\gamma` (dimensionless)
* :math:`R_m` (distance units)
* :math:`\epsilon` (energy units)
* cutoff (distance units)
The last coefficient is optional. If not specified, the global cutoff is used.
----------
Mixing, shift, table, tail correction, restart, rRESPA info
"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
This pair style does not support mixing. Thus, coefficients for all I,J
pairs must be specified explicitly.
This pair style supports the :doc:`pair_modify <pair_modify>` shift
option for the energy of the pair interaction.
The :doc:`pair_modify <pair_modify>` table options are not relevant for
this pair style.
This pair style does not support the :doc:`pair_modify <pair_modify>`
tail option for adding long-range tail corrections to energy and
pressure.
This pair style writes its 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.
This pair style supports the use of the *inner*, *middle*, and
*outer* keywords of the :doc:`run_style respa <run_style>` command,
meaning the pairwise forces can be partitioned by distance at different
levels of the rRESPA hierarchy. See the :doc:`run_style <run_style>`
command for details.
----------
Restrictions
""""""""""""
This pair style is only enabled if LAMMPS was built with the EXTRA-PAIR
package. See the :doc:`Build package <Build_package>` page for more
info.
Related commands
""""""""""""""""
* :doc:`pair_coeff <pair_coeff>`
* :doc:`pair_style lj/cut <pair_lj_cut>`
Default
"""""""
none
--------------
.. _Pirani:
**(Pirani)** F. Pirani, S. Brizi, L. Roncaratti, P. Casavecchia, D. Cappelletti and F. Vecchiocattivi, Phys. Chem. Chem. Phys., 2008, 10, 5489–5503.
.. _Arroyo:
**(Arroyo)** E. García-Arroyo, J. Campos-Martínez, M. Bartolomei, F. Pirani and M. Hernández, Phys. Chem. Chem. Phys., 2022,24, 15840-15850
.. _Chatzigeorgis:
**(Chatzigeorgis)** G. Bellas-Chatzigeorgis, J. Haskins and J. Scoggins , Physics of Fluids 1 August 2022; 34 (8): 087106.