# pair_style gran command

# pair_style bubble command

# pair_style gran_bubble bubble command

## Syntax

    pair_style gran model_type model_name model_keyword model_value

    pair_style bubble model_type model_name model_keyword model_value

    pair_style gran_bubble model_type model_name model_keyword model_value

- zero or more model_type/model_name pairs may be appended. They must be appended in the following order (!)

    model values = described here
    tangential values = described here
    cohesion values = described here
    rolling_friction values = described here
    surface values = described here

- following the model_type/model_name pairs, zero or more model_keyword/model_valuezero pairs may be appended in arbitrary order

    model_type/model_name pairs = described for each model separately here

## Examples

    pair_style gran model hooke tangential history
    pair_style gran model hertz tangential history rolling_friction cdt
    pair_style gran model hertz tangential no_history cohesion sjkr

General description:

The gran styles imposes a force between two neighboring particles. Typically, there is a force when the distance r between two particles of radii Ri and Rj is less than their contact distance dist = Ri + Rj, and no force otherwise. Some models, such as cohesion models, may impose a force also when the particle surfaces do not touch. This is documented for those models specifically.

The general form of a granular interaction can be written as:

The quantities in the equations are as follows:

- delta_n = d - r = overlap distance of 2 particles
- k_n = elastic constant for normal contact
- k_t = elastic constant for tangential contact
- gamma_n = viscoelastic damping constant for normal contact
- gamma_t = viscoelastic damping constant for tangential contact
- delta_t = tangential displacement vector between 2 spherical particles

In the first term is the normal force between the two particles and the second term is the tangential force. The normal force has 2 terms, a contact force and a damping force. The tangential force also has 2 terms: a shear force and a damping force. The shear force is a “history” effect that accounts for the tangential displacement (“tangential overlap”) between the particles for the duration of the time they are in contact.

The concrete implementation for k_n, k_t, gamma_n, gamma_t and the shear history depend on the concrete models as chosen by the user. They are described on separate doc pages here

Also, other models may add additional forces or torques on the particles, such as cohesive or rolling friction forces. These are also described on separate doc pages here

The styles bubble and gran_bubble are aliases for gran, which can e.g. be used for the modelling of systems with different phases using pair hybrid, where a different set of interaction laws is used for each phase. An example would be

    pair_style hybrid gran        model hertz     tangential history &
                      bubble      model hertz     tangential off &
                      gran_bubble model hertz     tangential off

Warning

The order of model keywords is important, you have to stick to the order as outlined in the “Syntax” section of this doc page.

General comments:

For granular styles there are no additional coefficients to set for each pair of atom types via the pair_coeff command. All settings are global and are made via the pair_style command. However you must still use the pair_coeff for all pairs of granular atom types. For example the command

    pair_coeff * *

should be used if all atoms in the simulation interact via a granular potential (i.e. one of the pair styles above is used). If a granular potential is used as a sub-style of pair_style hybrid, then specific atom types can be used in the pair_coeff command to determine which atoms interact via a granular potential.

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

The `pair_modify` mix, shift, table, and tail options are not relevant for granular pair styles.

These pair styles write their information to binary restart files, so a pair_style command does not need to be specified in an input script that reads a restart file.

Warning

The material properties are not written to restart files! Thus, if you restart a simulation, you have to re-define them (by using the fixes mentioned above).

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

These pair styles require that atoms store torque and angular velocity (omega) as defined by the atom_style. They also require a per-particle radius is stored. The sphere or granular atom style does all of this.

This pair style requires you to use the communicate vel yes option so that velocites are stored by ghost atoms.

Only unit system that are self-consistent (si, cgs, lj) can be used with this pair style.

## Related commands

pair_coeff Models for use with this command are described here

## Default

model = ‘hertz’ tangential = ‘off’ rolling_friction = ‘off’ cohesion = ‘off’ surface = ‘default’