# fix multicontact/halfspace command

## Syntax

    fix ID group multicontact/halfspace geometric_prefactor gp_value

- ID, group are documented in fix command
- multicontact/halfspace = style name of this fix command
- geometric_prefactor/gp_value = optional keyword-value pair

    geometric_prefactor gp_value = gamma
      gamma is an empirical factor accounting for the geometry

## Examples

    fix mc all multicontact/halfspace
    fix mc all multicontact/halfspace geometric_prefactor 1.8

## Description

Implements the multicontact model by (Brodu et al.) . This model computes a per-contact deformation for each particle based on the other contacts this particles has. A particle i with contact ij has the following new radius (when computing the contact laws with particle j):

    r_i + sum_k delta_{ij->ik}

where r_i is the default radius of particle i, sum_k is the sum over all particles in contact with i (k!=*j*). The delta value is given by

    delta_{ij->ik} = - gamma (1 + nu) F_ik / (2 pi Y d_{ik->ij}) * [(**n**_ik . **u**_{ik->ij})(**n**_ij . **u**_{ik->ij}) + (3 - 4 nu) **n**_ik . **n**_ij - (1 - 2 nu) (**n**_ik + **u**_{ik->ij}) . **n**_ij) / (1 + **n**_ik **u**_{ik->ij}) ]

where

- gamma = geometric prefactor
- nu = Poisson ratio
- F_ik = absolute value of normal force acting at contact ik
- Y = Youngs modulus
- d_{ik->ij} = distance from contact ik to contact ij
- **u**_{ik->ij} = unit vector pointing from contact ik to contact ij
- **n**_ik = normal vector of contact ik
- **n**_ij = normal vector of contact ij

## Restart, fix_modify, output, run start/stop, minimize info

No information about this fix is written to binary restart files.

fix_modify cannot be used on the parameter of this fix.

## Restrictions

Requires the use of the gran surface model multicontact

## Related commands

gran surface multicontact

Default: none

References:

(Brodu) Brodu, Dijksman, Behringer, Multiple-contact discrete-element model for simulating dense granular media, Phys. Rev. E (2015).