\(\renewcommand{\AA}{\text{Å}}\)

# pair_style rheo/solid command

## Syntax

    pair_style rheo/solid

## Examples

    pair_style rheo/solid
    pair_coeff * * 1.0 1.5 1.0

## Description

Added in version 29Aug2024.

Style rheo/solid is effectively a copy of pair style bpm/spring except it only applies forces between solid RHEO particles, determined by checking the status of each pair of neighboring particles before calculating forces.

The style computes pairwise forces with the formula

\[F = k (r - r_c)\]

where \(k\) is a stiffness and \(r_c\) is the cutoff length. An additional damping force is also applied to interacting particles. The force is proportional to the difference in the normal velocity of particles

\[F_D = - \gamma w (\hat{r} \bullet \vec{v})\]

where \(\gamma\) is the damping strength, \(\hat{r}\) is the displacement normal vector, \(\vec{v}\) is the velocity difference between the two particles, and \(w\) is a smoothing factor. This smoothing factor is constructed such that damping forces go to zero as particles come out of contact to avoid discontinuities. It is given by

\[w = 1.0 - \left( \frac{r}{r_c} \right)^8 .\]

The following coefficients must be defined for each pair of atom 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, or by mixing as described below:

- \(k\) (force/distance units)

- \(r_c\) (distance units)

- \(\gamma\) (force/velocity units)

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

For atom type pairs I,J and I != J, the A coefficient and cutoff distance for this pair style can be mixed. A is always mixed via a geometric rule. The cutoff is mixed according to the pair_modify mix value. The default mix value is geometric. See the “pair_modify” command for details.

This pair style does not support the pair_modify shift option, since the pair interaction goes to 0.0 at the cutoff.

The pair_modify table and tail options are not relevant for this pair style.

This pair style writes its 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.

This pair style can only be used via the pair keyword of the run_style respa command. It does not support the inner, middle, outer keywords.

## Restrictions

This pair style is part of the RHEO package. It is only enabled if LAMMPS was built with that package. See the Build package page for more info.

## Related commands

fix rheo, fix rheo/thermal, pair bpm/spring

## Default

none