# fix_modify AtC control

## syntax

fix_modify AtC control <physics_type> <solution_parameter>

- physics_type (string) = thermal | momentum
- solution_parameter (string) = max_iterations | tolerance

fix_modify AtC transfer <physics_type> control max_iterations <max_iterations>

- max_iterations (int) = maximum number of iterations that will be used by iterative matrix solvers

fix_modify AtC transfer <physics_type> control tolerance <tolerance>

- tolerance (float) = relative tolerance to which matrix equations will be solved

## examples

`fix_modify AtC control thermal max_iterations 10`
`fix_modify AtC control momentum tolerance 1.e-5`

## description

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.

## restrictions

only for be used with specific controllers : thermal, momentum
They are ignored if a lumped solution is requested

## related

## default

max_iterations is the number of rows in the matrix
tolerance is 1.e-10