# fix_modify AtC control momentum command

## Syntax

    fix_modify <AtC fixID> control <physics_type> <solution_parameter> <value>
    fix_modify AtC control momentum none
    fix_modify AtC control momentum rescale <frequency>
    fix_modify AtC control momentum glc_displacement
    fix_modify AtC control momentum glc_velocity
    fix_modify AtC control momentum hoover
    fix_modify AtC control momentum flux [faceset face_set_id, interpolate]

-   AtC fixID = ID of [fix atc](fix_atc) instance
-   control = name of the AtC sub-command
-   physics_type = *thermal* or *momentum*
-   solution_parameter = *max_iterations* or *tolerance*
-   value = solution_parameter value
-   momentum option = *none* or *rescale* or *glc_displacement* or
    *glc_velocity* *hoover* or *flux*
-   frequency = time step frequency for applying displacement and
    velocity rescaling
-   faceset_id = id of boundary face set (optional, only for *faceset*)

## Examples

``` LAMMPS
fix_modify AtC control momentum none
fix_modify AtC control momentum flux faceset bndy_faces
fix_modify AtC control momentum glc_velocity
```

## Description

The general version of *control* sets the numerical parameters for the
matrix solvers used in the specified control algorithm. Many solution
approaches require iterative solvers, and these methods enable users to
provide the maximum number of iterations and the relative tolerance.

The *control momentum* version sets the momentum exchange mechanism from
the finite elements to the atoms, managed through a control algorithm.
*rescale* computes a scale factor for each atom to match the finite
element temperature. *hoover* is a Gaussian least-constraint isokinetic
thermostat enforces that the nodal restricted atomic temperature matches
the finite element temperature. *flux* is a similar mode, but rather
adds energy to the atoms based on conservation of energy.

*correction_max_iterations* sets the maximum number of iterations to
compute the second order in time correction term for lambda with the
fractional step method. The method uses the same tolerance as the
controller\'s matrix solver.

## Restrictions

Only for be used with the specific controllers *thermal* or *momentum*.
They are ignored if a lumped solution is requested.

*control momentum* is only for be used with specific transfers: elastic
*rescale* not valid with time filtering activated

## Related AtC commands

-   [fix_modify AtC command overview](atc_fix_modify)
-   [fix_modify AtC control thermal](atc_control_thermal)

## Default

-   *max_iterations* is the number of rows in the matrix.
-   *tolerance* is 1.0e-10.