# pair_style smd/ulsph command

## Syntax

``` LAMMPS
pair_style smd/ulsph args
```

-   these keywords must be given

<!-- -->

    keyword = *\*DENSITY_SUMMATION* or *\*DENSITY_CONTINUITY* and *\*VELOCITY_GRADIENT* or *\*NO_VELOCITY_GRADIENT* and *\*GRADIENT_CORRECTION* or *\*NO_GRADIENT_CORRECTION*

## Examples

``` LAMMPS
pair_style smd/ulsph *DENSITY_CONTINUITY *VELOCITY_GRADIENT *NO_GRADIENT_CORRECTION
```

## Description

The *smd/ulsph* style computes particle interactions according to
continuum mechanics constitutive laws and an updated Lagrangian
Smooth-Particle Hydrodynamics algorithm.

This pair style is invoked similar to the following command:

``` LAMMPS
pair_style smd/ulsph *DENSITY_CONTINUITY *VELOCITY_GRADIENT *NO_GRADIENT_CORRECTION
pair_coeff i j *COMMON rho0 c0 Q1 Cp hg &
               *END
```

Here, *i* and *j* denote the *LAMMPS* particle types for which this pair
style is defined. Note that *i* and *j* can be different, i.e., *ulsph*
cross interactions between different particle types are allowed.
However, *i*\--\*i\* respectively *j*\--\*j\* pair_coeff lines have to
precede a cross interaction. In contrast to the usual *LAMMPS* *pair
coeff* definitions, which are given solely a number of floats and
integers, the *ulsph* *pair coeff* definition is organized using
keywords. These keywords mark the beginning of different sets of
parameters for particle properties, material constitutive models, and
damage models. The *pair coeff* line must be terminated with the *\*END*
keyword. The use the line continuation operator *&* is recommended. A
typical invocation of the *ulsph* for a solid body would consist of an
equation of state for computing the pressure (the diagonal components of
the stress tensor), and a material model to compute shear stresses (the
off-diagonal components of the stress tensor).

Note that the use of \*GRADIENT_CORRECTION can lead to severe numerical
instabilities. For a general fluid simulation, \*NO_GRADIENT_CORRECTION
is recommended.

Please see the [SMD user guide](PDF/MACHDYN_LAMMPS_userguide.pdf)\_ for
a complete listing of the possible keywords and material models.

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

## 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