# fix_modify command

## Syntax

    fix_modify fix-ID keyword value ...

- fix-ID = ID of the fix to modify
- one or more keyword/value pairs may be appended
- keyword = temp or press or energy

    temp value = compute ID that calculates a temperature
    press value = compute ID that calculates a pressure
    energy value = yes or no

## Examples

    fix_modify 3 temp myTemp press myPress
    fix_modify 1 energy yes

## Description

Modify one or more parameters of a previously defined fix. Only specific fix styles support specific parameters. See the doc pages for individual fix commands for info on which ones support which fix_modify parameters.

The temp keyword is used to determine how a fix computes temperature. The specified compute ID must have been previously defined by the user via the compute command and it must be a style of compute that calculates a temperature. All fixes that compute temperatures define their own compute by default, as described in their documentation. Thus this option allows the user to override the default method for computing T.

The press keyword is used to determine how a fix computes pressure. The specified compute ID must have been previously defined by the user via the compute command and it must be a style of compute that calculates a pressure. All fixes that compute pressures define their own compute by default, as described in their documentation. Thus this option allows the user to override the default method for computing P.

For fixes that calculate a contribution to the potential energy of the system, the energy keyword will include that contribution in thermodynamic output of potential energy. See the thermo_style command for info on how potential energy is output. The contribution by itself can be printed by using the keyword f_ID in the thermo_style custom command, where ID is the fix-ID of the appropriate fix. Note that you must use this setting for a fix if you are using it when performing an `energy minimization` and if you want the energy and forces it produces to be part of the optimization criteria.

## Restrictions

> none

## Related commands

fix, `compute temp`, compute pressure, thermo_style

## Default

The option defaults are temp = ID defined by fix, press = ID defined by fix, energy = no.