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\(\renewcommand{\AA}{\text{Å}}\)

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pair_style ilj/cut command

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Syntax

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pair_style ilj/cut cutoff
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  • ilj/cut = name of the pair style

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  • cutoff = global cutoff (distance units)

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Examples

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pair_style ilj/cut 10.0
-pair_coeff 1 1 4.0 7.0 6.0 3.5 0.0045
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Description

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New in version 29Aug2024.

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Pair style ilj/cut computes pairwise interactions from an Improved Lennard-Jones (ILJ) potential according to (Pirani):

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- - \[x = r/R_m \] - -
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- - \[n_x = \alpha \cdot x^2 + \beta \] - -
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- - \[\gamma = m \] - -
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- - \[V(x) = \epsilon \cdot \left( \frac{\gamma}{n_x - \gamma} \left(\frac{1}{x} \right)^{n_x} - \frac{n_x}{n_x - \gamma} \left(\frac{1}{x} \right)^{\gamma} \right) - \qquad r < r_c\]
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\(r_c\) is the cutoff.

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- An additional parameter, \(\alpha\) , has been introduced in order to - be able to recover the traditional LJ 12-6 with an adequate choice of parameters. With - \( R_m \equiv r_0 = \sigma \cdot 2^{1/6} \) , - \(\alpha = 0\) , \(\beta = 12\) and \(\gamma = 6\) it is straightforward - to prove that LJ 12-6 is obtained. - -

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- - 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 (Pirani). It can be used for neutral-neutral ( \(\gamma = 6\) ), ion-neutral ( \(\gamma = 4\) ) or ion-ion systems ( \(\gamma = 1\) ). It removes most of the issues at short- and long-range of the LJ model. -

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- - It is possible to verify that using \(\alpha = 4\), \(\beta = 8\) and \(\gamma = 6\), 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. - -

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- - The analysis of a diverse amount of systems verified that \(\alpha = 4\) 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 \(\beta\) codifies the hardness (polarizability) of the interacting partners, and for neutral-neutral systems it ranges from 6 to 11. Moreover, the modulation of \(\beta\) permits to indirectly consider the role of further interaction components (such as the charge transfer in the perturbative limit) and mitigates the effect of some uncertainty in the data. - -

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- - A couple of examples of the applications of the ILJ potential should be useful to show its wide variety of uses: -

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  • The ILJ potential has been successfully used to model the interaction between a graphdiyne (GDY) membrane and molecular hydrogen (Arroyo et al).

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  • The ILJ model and the phenomenological approach have been recently adopted by the NASA Laboratory to evaluate transport properties in gaseous mixtures (Chatzigeorgis et al).

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

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  • \( \alpha \) (dimensionless)

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  • \(\beta\) (dimensionless)

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  • \(\gamma\) (dimensionless)

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  • \( R_m \) (distance units)

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  • \( \epsilon \) (energy units)

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  • cutoff (distance units)

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

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

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This pair style does not support mixing. Thus, coefficients for all I,J -pairs must be specified explicitly.

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This pair style supports the pair_modify shift -option for the energy of the pair interaction.

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The pair_modify table options are not relevant for -this pair style.

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This pair style does not support the pair_modify -tail option for adding long-range tail corrections to energy and -pressure.

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This pair style writes its information to binary restart files, so pair_style and pair_coeff commands do not need to be -specified in an input script that reads a restart file.

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The ilj/cut pair styles support the use of the inner, middle, and - outer keywords of the run_style respa command, - meaning the pairwise forces can be partitioned by distance at different - levels of the rRESPA hierarchy. See the run_style - command for details.

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Restrictions

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This pair style is only enabled if LAMMPS was built with the EXTRA-PAIR -package. See the Build package page for more -info.

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Default

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none

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(Pirani) F. Pirani, S. Brizi, L. Roncaratti, P. Casavecchia, D. Cappelletti and F. Vecchiocattivi, Phys. Chem. Chem. Phys., 2008, 10, 5489–5503.

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(Arroyo) E. García-Arroyo, J. Campos-Martínez, M. Bartolomei, F. Pirani and M. Hernández, Phys. Chem. Chem. Phys., 2022,24, 15840-15850

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(Chatzigeorgis) G. Bellas-Chatzigeorgis, J. Haskins and J. Scoggins , Physics of Fluids 1 August 2022; 34 (8): 087106.

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