\(\renewcommand{\AA}{\text{Å}}\)

pair_style surf/granular command

Syntax

pair_style surf/granular cutoff

cutoff = global cutoff (optional). See discussion below.

Examples

pair_style surf/granular
pair_coeff * * hooke 1000.0 50.0 tangential linear_nohistory 1.0 0.4 damping mass_velocity

pair_style surf/granular
pair_coeff * * hooke 1000.0 50.0 tangential linear_history 500.0 1.0 0.4 damping mass_velocity

Description

Added in version TBD.

The surf/granular pair style is designed for interactions between granular surfaces made out of lines/triangles and finite-sized atoms and must be used in conjunction with fix surface/local. See the Howto granular surfaces doc page for more information on granular surfaces.

The equation for the force between a triangle/line and a particle touching it is the same as the corresponding equation on the pair_style granular doc page in the limit of one of the two particles going to infinite radius and mass (flat surface). Specifically, \(\delta\) = overlap of particle with triangle/line, \(m_eff\) = mass of particle, and the effective radius of contact R_{eff}= R_i R_j/R_i + R_j is set to the radius of the particle. See the Howto granular surfaces doc page for information on how overlaps and normal vectors are calculated based on the geometry of the surface and when friction is transferred between lines/triangles.

All model choices and parameters are entered in the pair_coeff command. Coefficient values are not global, but can be set to different values for different combinations of particle types, as determined by the pair_coeff command. If the contact model choice is the same for two particle types, the mixing for the cross-coefficients can be carried out automatically. For additional flexibility, coefficients as well as model forms can vary between particle types. Available pair coefficients and contact models are identical to those in the granular pair style. The only exception is that forces cannot extend beyond contact as in the JKR contact model.

LAMMPS automatically sets pairwise cutoff values for pair_style granular based on the atom radii. In the vast majority of situations, this is adequate. However, a cutoff value can optionally be appended to the pair_style granular command to specify a global cutoff (i.e. a cutoff for all atom types). Additionally, the optional cutoff keyword can be passed to the pair_coeff command, followed by a cutoff value. This will set a pairwise cutoff for the atom types in the pair_coeff command. These options may be useful in some rare cases where the automatic cutoff determination is not sufficient, e.g. if particle diameters are being modified via the fix adapt command. In that case, the global cutoff specified as part of the pair_style granular command is applied to all atom types, unless it is overridden for a given atom type combination by the cutoff value specified in the pair coeff command. If cutoff is only specified in the pair coeff command and no global cutoff is appended to the pair_style granular command, then LAMMPS will use that cutoff for the specified atom type combination, and automatically set pairwise cutoffs for the remaining atom types.

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

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

Mixing of coefficients is carried out using geometric averaging for most quantities. See the doc page for pair granular discussion of exceptions.

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.

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.

The single() function of these pair styles returns 0.0 for the energy of a pairwise interaction, since energy is not conserved in these dissipative potentials. It also returns only the normal component of the pairwise interaction force. However, the single() function also calculates extra pairwise quantities. See the doc page for pair granular for additional information on what these quantities are and how to access them.

Restrictions

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

This pair style must be in used in conjunction with fix surface/local.

This pair style requires the data structures in atom_style line in 2d and atom_style line in 3d. Additionally, this pair style requires that atoms store per-particle radius, torque, and angular velocity (omega). These quantities are defined by the line/tri atom styles, however, the radii are not written to data files so it is recommended to hybridize these atom styles with atom style sphere using atom style hybrid.

This pair style requires you to use the comm_modify vel yes command so that velocities are stored by ghost atoms.

This pair style will not restart exactly when using the read_restart command, though it should provide statistically similar results. This is because the forces it computes depend on atom velocities and the atom velocities have been propagated half a timestep between the force computation and when the restart is written, due to using Velocity Verlet time integration. See the read_restart command for more details.

Accumulated values for individual contacts are saved to restart files but are not saved to data files. Therefore, forces may differ significantly when a system is reloaded using the read_data command.

Default

For the pair_coeff settings: damping viscoelastic, rolling none, twisting none.