# pair_style smd/hertz command

## Syntax

``` LAMMPS
pair_style smd/hertz scale_factor
```

## Examples

``` LAMMPS
pair_style smd/hertz 1.0
pair_coeff 1 1 <contact_stiffness>
```

## Description

The *smd/hertz* style calculates contact forces between SPH particles
belonging to different physical bodies.

The contact forces are calculated using a Hertz potential, which
evaluates the overlap between two particles (whose spatial extents are
defined via its contact radius). The effect is that a particles cannot
penetrate into each other. The parameter \<contact_stiffness\> has units
of pressure and should equal roughly one half of the Young\'s modulus
(or bulk modulus in the case of fluids) of the material model associated
with the SPH particles.

The parameter *scale_factor* can be used to scale the particles\'
contact radii. This can be useful to control how close particles can
approach each other. Usually, *scale_factor* =1.0.

------------------------------------------------------------------------

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

No mixing is performed automatically. Currently, no part of MACHDYN
supports restarting nor minimization. rRESPA does not apply to this pair
style.

------------------------------------------------------------------------

## Restrictions

This fix is part of the MACHDYN package. It is only enabled if LAMMPS
was built with that package. See the [Build package](Build_package) page
for more info.

## Related commands

[pair_coeff](pair_coeff)

## Default

none