\(\renewcommand{\AA}{\text{Å}}\)

pair_style lj/mdf command

Accelerator Variant: lj/mdf/omp

pair_style buck/mdf command

Accelerator Variant: buck/mdf/omp

pair_style lennard/mdf command

Accelerator Variant: lennard/mdf/omp

Syntax

pair_style style args

  • style = lj/mdf or buck/mdf or lennard/mdf

  • args = list of arguments for a particular style

    lj/mdf args = cutoff1 cutoff2
  cutoff1 = inner cutoff for the start of the tapering function
  cutoff1 = out cutoff for the end of the tapering function
buck/mdf args = cutoff1 cutoff2
  cutoff1 = inner cutoff for the start of the tapering function
  cutoff1 = out cutoff for the end of the tapering function
lennard/mdf args = cutoff1 cutoff2
  cutoff1 = inner cutoff for the start of the tapering function
  cutoff1 = out cutoff for the end of the tapering function

Examples

pair_style lj/mdf 2.5 3.0
pair_coeff * * 1.0 1.0
pair_coeff 1 1 1.1 2.8 3.0 3.2

pair_style buck/mdf 2.5 3.0
pair_coeff * * 100.0 1.5 200.0
pair_coeff * * 100.0 1.5 200.0 3.0 3.5

pair_style lennard/mdf 2.5 3.0
pair_coeff * * 1.0 1.0
pair_coeff 1 1 1021760.3664 2120.317338 3.0 3.2

Description

The lj/mdf, buck/mdf and lennard/mdf compute the standard 12-6 Lennard-Jones and Buckingham potential with the addition of a taper function that ramps the energy and force smoothly to zero between an inner and outer cutoff.

\[E_{smooth}(r) = E(r)*f(r)\]

The tapering, f(r), is done by using the Mei, Davenport, Fernando function (Mei).

\[\begin{split}f(r) & = 1.0 \qquad \qquad \mathrm{for} \qquad r < r_m \\ f(r) & = (1 - x)^3*(1+3x+6x^2) \quad \mathrm{for} \qquad r_m < r < r_{cut} \\ f(r) & = 0.0 \qquad \qquad \mathrm{for} \qquad r >= r_{cut} \\\end{split}\]

where

\[x = \frac{(r-r_m)}{(r_{cut}-r_m)}\]

Here \(r_m\) is the inner cutoff radius and \(r_{cut}\) is the outer cutoff radius.

For the lj/mdf pair_style, the potential energy, E(r), is the standard 12-6 Lennard-Jones written in the epsilon/sigma form:

\[E(r) = 4 \epsilon \left[ \left(\frac{\sigma}{r}\right)^{12} - \left(\frac{\sigma}{r}\right)^6 \right]\]

Either the first two or all of 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 read by the read_data. The two cutoffs default to the global values and \(\epsilon\) and \(\sigma\) can also be determined by mixing as described below:

  • \(\epsilon\) (energy units)

  • \(\sigma\) (distance units)

  • \(r_m\) (distance units)

  • \(r_{cut}\) (distance units)

For the buck/mdf pair_style, the potential energy, E(r), is the standard Buckingham potential with three required coefficients. The two cutoffs can be omitted and default to the corresponding global values:

\[E(r) = A e^{(-r/\rho)} -\frac{C}{r^6}\]

  • A (energy units)

  • \(\rho\) (distance units)

  • C (energy-distance^6 units)

  • \(r_m\) (distance units)

  • \(r_{cut}\) (distance units)

For the lennard/mdf pair_style, the potential energy, E(r), is the standard 12-6 Lennard-Jones written in the A/B form:

\[E(r) = \frac{A}{r^{12}} - \frac{B}{r^{6}}\]

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 read by the read_data commands, or by mixing as described below. The two cutoffs default to their global values and must be either both given or both left out:

  • A (energy-distance^12 units)

  • B (energy-distance^6 units)

  • \(r_m\) (distance units)

  • \(r_{cut}\) (distance units)

Styles with a gpu, intel, kk, omp, or opt suffix are functionally the same as the corresponding style without the suffix. They have been optimized to run faster, depending on your available hardware, as discussed on the Accelerator packages page. The accelerated styles take the same arguments and should produce the same results, except for round-off and precision issues.

These accelerated styles are part of the GPU, INTEL, KOKKOS, OPENMP, and OPT packages, respectively. They are only enabled if LAMMPS was built with those packages. See the Build package page for more info.

You can specify the accelerated styles explicitly in your input script by including their suffix, or you can use the -suffix command-line switch when you invoke LAMMPS, or you can use the suffix command in your input script.

See the Accelerator packages page for more instructions on how to use the accelerated styles effectively.

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

For atom type pairs I,J and I != J, the \(\epsilon\) and \(\sigma\) coefficients and cutoff distances for the lj/mdf pair style can be mixed. The default mix value is geometric. See the “pair_modify” command for details. The other two pair styles buck/mdf and lennard/mdf do not support mixing, so all I,J pairs of coefficients must be specified explicitly.

None of the lj/mdf, buck/mdf, or lennard/mdf pair styles supports the pair_modify shift option or long-range tail corrections to pressure and energy.

These styles write their 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.

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

Restrictions

These pair styles can only be used if LAMMPS was built with the EXTRA-PAIR package. See the Build package doc page for more info.

Default

none

(Mei) Mei, Davenport, Fernando, Phys Rev B, 43 4653 (1991)