# fix_modify AtC time_integration command

## Syntax

    fix_modify <AtC fixID> time_integration <descriptor>

-   AtC fixID = ID of [fix atc](fix_atc) instance
-   time_integration = name of the AtC sub-command
-   descriptor = *gear* or *fractional_step* or *verlet*

## Examples

``` LAMMPS
fix_modify AtC time_integration fractional_step
```

## Description

Command to select the thermal or momentum time integration.

------------------------------------------------------------------------

Options for thermal time integration:

*gear*

:   atomic velocity update with second order Verlet, nodal temperature
    update with third or fourth order Gear, thermostats based on
    controlling power

*fractional_step*

:   atomic velocity update with second order Verlet, mixed nodal
    temperature update, 3/4 Gear for continuum and 2 Verlet for atomic
    contributions, thermostats based on controlling discrete energy
    changes

------------------------------------------------------------------------

Options for momentum time integration:

*verlet*

:   atomic velocity update with second order Verlet, nodal temperature
    update with second order Verlet, kinetostats based on controlling
    force

*fractional_step*

:   atomic velocity update with second order Verlet, mixed nodal
    momentum update, second order Verlet for continuum and exact second
    order Verlet for atomic contributions, kinetostats based on
    controlling discrete momentum changes

*gear*

:   atomic velocity update with second order Verlet, nodal temperature
    update with third or fourth order Gear, kinetostats based on
    controlling power.

------------------------------------------------------------------------

## Restrictions

None.

## Related AtC commands

-   [fix_modify AtC command overview](atc_fix_modify)

## Default

None.