# pair_style morse/kk command

## 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)
morse/soft args = n lf cutoff
n       = soft-core parameter
lf      = transformation range is lf < lambda < 1
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

pair_style morse/soft 4 0.9 10.0
pair_coeff * * 100.0 2.0 1.5 1.0
pair_coeff 1 1 100.0 2.0 1.5 1.0 3.0


## Description

Style morse computes pairwise interactions with the formula

Rc 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:

• D0 (energy units)
• alpha (1/distance units)
• r0 (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:

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

The morse/soft variant is similar to the lj/cut/soft pair style in that it modifies the potential at short range to have a soft core. This helps to avoid singularities during free energy calculation in which sites are created or annihilated. The formula differs from that of lj/cut/soft, and is instead given by:

The morse/soft style requires the following pair coefficients:

• D0 (energy units)
• alpha (1/distance units)
• r0 (distance units)
• lamda (unitless, between 0.0 and 1.0)
• cutoff (distance units)

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

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 in Section 5 of the manual. 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, USER-INTEL, KOKKOS, USER-OMP and OPT packages, respectively. They are only enabled if LAMMPS was built with those packages. See the Making LAMMPS section 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 Section 5 of the manual 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 is 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 USER-MISC package. See the Making LAMMPS section for more info.

The morse/soft pair style is only enabled if LAMMPS was built with the USER-FEP package. See the Making LAMMPS section for more info.