\(\renewcommand{\AA}{\text{Å}}\)

pair_style lj/switch3/coulgauss/long command

Accelerator Variants: lj/switch3/coulgauss/long/kk, lj/switch3/coulgauss/long/omp

pair_style mm3/switch3/coulgauss/long command

Accelerator Variants: mm3/switch3/coulgauss/long/kk, mm3/switch3/coulgauss/long/omp

Syntax

pair_style style args

  • style = lj/switch3/coulgauss/long or mm3/switch3/coulgauss/long

  • args = list of arguments for a particular style

lj/switch3/coulgauss/long args = cutoff (cutoff2) width
  cutoff  = global cutoff for LJ (and Coulombic if only 1 arg) (distance units)
  cutoff2 = global cutoff for Coulombic (optional) (distance units)
  width  = width parameter of the smoothing function (distance units)

mm3/switch3/coulgauss/long args = cutoff (cutoff2) width
  cutoff  = global cutoff for MM3 (and Coulombic if only 1 arg) (distance units)
  cutoff2 = global cutoff for Coulombic (optional) (distance units)
  width  = width parameter of the smoothing function (distance units)

Examples

pair_style lj/switch3/coulgauss/long    12.0 3.0
pair_coeff 1  0.2 2.5 1.2

pair_style lj/switch3/coulgauss/long   12.0 10.0 3.0
pair_coeff 1  0.2 2.5 1.2

pair_style mm3/switch3/coulgauss/long    12.0 3.0
pair_coeff 1  0.2 2.5 1.2

pair_style mm3/switch3/coulgauss/long   12.0 10.0 3.0
pair_coeff 1  0.2 2.5 1.2

Description

The lj/switch3/coulgauss style evaluates the LJ vdW potential

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

The mm3/switch3/coulgauss/long style evaluates the MM3 vdW potential (Allinger)

\[\begin{split}E & = \epsilon_{ij} \left[ -2.25 \left(\frac{r_{v,ij}}{r_{ij}}\right)^6 + 1.84(10)^5 \exp\left[-12.0 r_{ij}/r_{v,ij}\right] \right] S_3(r_{ij}) \\ r_{v,ij} & = r_{v,i} + r_{v,j} \\ \epsilon_{ij} & = \sqrt{\epsilon_i \epsilon_j}\end{split}\]

Both potentials go smoothly to zero at the cutoff r_c as defined by the switching function

\[\begin{split}S_3(r) = \left\lbrace \begin{array}{ll} 1 & \quad\mathrm{if}\quad r < r_\mathrm{c} - w \\ 3x^2 - 2x^3 & \quad\mathrm{if}\quad r < r_\mathrm{c} \quad\mathrm{with\quad} x=\frac{r_\mathrm{c} - r}{w} \\ 0 & \quad\mathrm{if}\quad r >= r_\mathrm{c} \end{array} \right.\end{split}\]

where w is the width defined in the arguments. This potential is combined with Coulomb interaction between Gaussian charge densities:

\[E = \frac{q_i q_j \mathrm{erf}\left( r/\sqrt{\gamma_1^2+\gamma_2^2} \right) }{\epsilon r_{ij}}\]

where \(q_i\) and \(q_j\) are the charges on the two atoms, \(\epsilon\) is the dielectric constant which can be set by the dielectric command, \(\gamma_i\) and \(\gamma_j\) are the widths of the Gaussian charge distribution and erf() is the error-function. This style has to be used in conjunction with the kspace_style command

If one cutoff is specified it is used for both the vdW and Coulomb terms. If two cutoffs are specified, the first is used as the cutoff for the vdW terms, and the second is the cutoff for the Coulombic term.

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:

  • \(\epsilon\) (energy)

  • \(\sigma\) (distance)

  • \(\gamma\) (distance)

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 distance for all of the lj/long pair styles can be mixed. The default mix value is geometric. See the “pair_modify” command for details.

Shifting the potential energy is not necessary because the switching function ensures that the potential is zero at the cut-off.

These pair styles support the pair_modify table and options since they can tabulate the short-range portion of the long-range Coulombic interactions.

Thes pair styles do not support the pair_modify tail option for adding a long-range tail correction to the Lennard-Jones portion of the energy and pressure.

These pair 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 pair 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 styles are part of the YAFF package. They are only enabled if LAMMPS was built with that package. See the Build package page for more info.

Default

none