pair_style lj/mdf command

pair_style buck/mdf command

pair_style lennard/mdf command

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_{s m o o t h} (r) = E (r) * f (r)

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

\begin{aligned} f (r) & = 1.0 for r < r_{m} \\ f (r) & = (1 - x)^{3} * (1 + 3 x + 6 x^{2}) for r_{m} < r < r_{c u t} \\ f (r) & = 0.0 for r >= r_{c u t} \end{aligned}

where

x = \frac{(r - r_{m})}{(r_{c u t} - r_{m})}

Here $r_{m}$ is the inner cutoff radius and $r_{c u t}$ 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 ϵ [{(\frac{σ}{r})}^{12} - {(\frac{σ}{r})}^{6}]

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 $ϵ$ and $σ$ can also be determined by mixing as described below:

$ϵ$ (energy units)
$σ$ (distance units)
$r_{m}$ (distance units)
$r_{c u t}$ (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 / ρ)} - \frac{C}{r^{6}}

A (energy units)
$ρ$ (distance units)
C (energy-distance^6 units)
$r_{m}$ (distance units)
$r_{c u t}$ (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)
$r_{c u t}$ (distance units)

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

For atom type pairs I,J and I != J, the $ϵ$ and $σ$ 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)