\(\renewcommand{\AA}{\text{Å}}\)

# pair_style morse command

Accelerator Variants: morse/gpu, morse/omp, morse/opt, morse/kk

# pair_style morse/smooth/linear command

Accelerator Variants: morse/smooth/linear/omp

## Syntax

    pair_style style args

- style = morse or morse/smooth/linear or morse/soft

- args = list of arguments for a particular style

    morse args = cutoff
      cutoff = global cutoff for Morse interactions (distance units)
    morse/smooth/linear args = cutoff
      cutoff = global cutoff for Morse interactions (distance units)

## Examples

    pair_style morse 2.5
    pair_style morse/smooth/linear 2.5
    pair_coeff * * 100.0 2.0 1.5
    pair_coeff 1 1 100.0 2.0 1.5 3.0

## Description

Style morse computes pairwise interactions with the formula

\[E = D_0 \left[ e^{- 2 \alpha (r - r_0)} - 2 e^{- \alpha (r - r_0)} \right] \qquad r < r_c\]

\(r_c\) is the cutoff.

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:

- \(D_0\) (energy units)

- \(\alpha\) (1/distance units)

- \(r_0\) (distance units)

- cutoff (distance units)

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

The morse/smooth/linear variant is similar to the lj/smooth/linear variant in that it adds to the potential a shift and a linear term so that both, potential energy and force, go to zero at the cut-off:

\[\begin{split}\phi\left(r\right) & = D_0 \left[ e^{- 2 \alpha (r - r_0)} - 2 e^{- \alpha (r - r_0)} \right] \qquad r < r_c \\ E\left(r\right) & = \phi\left(r\right) - \phi\left(r_c\right) - \left(r - r_c\right) \left.\frac{d\phi}{d r} \right|_{r=r_c} \qquad r < r_c\end{split}\]

The syntax of the pair_style and pair_coeff commands are the same for the morse and morse/smooth/linear styles.

A version of the morse style with a soft core, morse/soft, suitable for use in free energy calculations, is part of the FEP package and is documented with the pair_style */soft styles. The version with soft core is only available if LAMMPS was built with that package. See the Build package page for more info.

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

None of these pair styles support mixing. Thus, coefficients for all I,J pairs must be specified explicitly.

All of these pair styles support the pair_modify shift option for the energy of the pair interaction.

The pair_modify table options are not relevant for the Morse pair styles.

None of these pair styles support the pair_modify tail option for adding long-range tail corrections to energy and pressure.

All of 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

The morse/smooth/linear pair style is only enabled if LAMMPS was built with the EXTRA-PAIR package. See the Build package page for more info.

## Related commands

pair_coeff, pair_style */soft

## Default

none